Hartnell,
I tend to think of as having more weaknesses than strengths. He had the frailties of age, and by design he was not originally meant to be the hero of the show. That was meant to be Ian. Hartnell's Doctor was typically not interested in being the hero. Only in surviving and protecting Susan.
But he tended to have the wisest idea of what was to be done, and is usually shown to be proven right when his companions think against his advice, they can do their idea of the right thing to change history, and it blows up in their faces. He also had a formiddably ruthless intellect.
In a sense though, his era was less about seeing the Doctor in action, and more about seeing an anti-hero gradually *become* the Doctor we know. By the end of The Daleks' Masterplan, there's no doubt anymore that he is now the crusader for justice we know.
Troughton.
Much more committed to fighting the forces of evil, and with more experience of how Daleks and Cybermen think and how to fight them best. Retains Hartnell's ruthless intellect and in many ways amplifies it (ruthless in a way that sometimes made him closer to an Al-Qaeda terrorist when concerning the Daleks). He had more persuasive skills to influence others to realize the need to do the necessary thing.
I really can't think of notable weaknesses he might've had, aside from his fallible moment of overconfidence in Evil of the Daleks when bragging to the Emperor that he has sowed rebellion, only to be told the Daleks had been manipulating him all along to help them discover their own nature. And maybe in Enemy of the World he was a bit too late to be convinced of Salamander's true evil nature.
It has been pointed out that the ratings of his era were in something of a freefall. I don't think this was Trougton's fault, or the fault of the writers. I would venture that possibly there was a burnout of interest that Troughton sadly inherited. There might also have been parental concerns the show was getting too frightening for their children after the death of Katarina and Sara Kingdom.
Pertwee.
A few weaknesses stand out for me with this incarnation. Namely that he was unfortunately the first to set the template for the Doctor having moments of being an insufferable pacifist do-gooder (particularly concerning the Master), and he could be a bit of a head-patting fuddy-duddy in general. He also lacked the temperament for being an effective diplomat.
I think he brought a lot of urgency to the role at the beginning, but as his Doctor-Master rivalry continued, he seemed to become a bit too cozy and even easily gullible in the presence of his enemy playing the obvious 'reformed man' act. Frontier in Space particularly so.
Strengths were that he generally was still the smartest man in the room with the wisest approach. He took no shit and always maintained an air of dignity and pragmatism. He had a winning humor, twinkle and charisma without ever sending up the part.
Tom Baker.
In some ways he combined all the best elements of the Doctors before him. The physicality of Jon, the ruthless intellect and pragmatism of Troughton, and furious conviction of Hartnell.
Sometimes he did have his moments of absent-minded negligence however. Planet of Evil and Revenge of the Cybermen particularly come to mind (in the time it took him to walk to a dying Warner, he could've explained via radio how to save him via the transmat). But for the most part when he left an adventure, there was a sense that world had been immensely better for his presence than not.
For those three years with Hinchcliffe in charge, Tom's Doctor had been perfectly in his element. Then with the stormy sea-change to Williams, suddenly he wasn't quite in synch. I don't agree with the suggestion that he stopped caring, but it was more like the horse bolted, and he kept wanting to do his own thing based on what he was convinced would reflect the Doctor's eccentricity and entertain the kids, and was perhaps growing disillusioned with how the makers were taking the show. But often he really needed someone to clamp down on him a bit and make him commit to the script and tone at hand.
Even as late as City of Death though, he could still be a Doctor you wanted to be with all the way, and there was still something magical and life-affirming about his company and his approach. I think if there was a point I started to dislike him as the Doctor it was probably Nightmare of Eden. There are some moments there where he's downright punchable.
Unfortunately, however I cannot excuse any of the idiocy, ineptitude and incompetence he finally displayed in Logopolis. And worse is that it pretty much ended his era by undoing huge chunks of his good planet-saving work over his era. Sometimes I try not to count it as part of his era (it barely feels like a Tom story to begin with). But for the purposes of this discussion it is a bloody blow to his track record.
Davison.
To be fair to Davison, he did bring a lot of heart and commitment to the show, and for his first six stories it did seem like he was going to be a winner of a Doctor, and just the change the show needed. Contrary to fan opinion I think it should've been easy for the audience to take to him after Tom Baker. If anything the audience should've been rooting for him to avenge his predecessor from the start, and in Castrovalva he did.
Unfortunately that victory was inexplicably undone by Time-Flight and remained so for the rest of his era. And that was the problem I think with the rest of his era. It was easy to get behind Davison's preoccupation with small-stake concerns in Kinda and Black Orchid, so long as he believed the Master was vanquished. After he realized otherwise in Time-Flight, but didn't seem to care (or indeed seemed contractually obliged not to), it became a lot harder to emotionally invest
Sadly I think Davison was stagnated by the obsessive focus on continuity that seemed to require him to just imitate a more sanctimonious version of Pertwee, rather than giving him a chance to discover his own Doctor. Unfortunately I think he was a tragic case of an unfit for purpose production team, reimagining the Doctor in their own clueless, incompetent image. It was as if certain superfans who were calling the shots would rather the show remain faithful to the Doctor's stupidest stances in the past than allow him to grow wiser.
By Tom Baker's second season, had the show come to a stop there, Tom would've still felt like he'd enjoyed a solid, complete era. By Davison's second season he still felt half-formed, and a bit rote. And worse, he just felt like a sanctimonious do-gooder fuddy duddy. In short, all the worst, stupidest things the audience believe the Doctor to be.
But as poor a run as Season 20 was, there was still a chance for him to pull his socks up in Season 21 and redeem his crown. Unfortunately he did the opposite. A lot of fans say he had more authority in that season. But the problem is that's largely because he spends much of Season 21 being an avatar for Ian Levine.
I'll just say for now that Warriors of the Deep was the worst, most unforgivable exhibition of non-stop criminal negligence by any Doctor (only Orphan 55 comes even close). At that point I just hated his Doctor, and I very easily needn't have done.
It was especially sad, because often it would only have taken a few rewrites more (or in the case of Warriors of the Deep, a few rewrites less) to salvage his character's credibility.
By Resurrection of the Daleks it didn't even feel like his show anymore, and I have to say that's why I lost heart and didn't see the point in all the gratuitous nastiness in search of a tacked on moral virtue signal at the end. Caves of Androzani was sadly a taunting proof of his greater potential and what could've been if he'd started on that note rather than ended on it.
But for the most part, I think he almost erased any sense of character or nuance, or even common sense the character ever had. He was not a Doctor I would've trusted an inch with my life, and the fact his companions did just exposed the soulless artifice of the era for me. And even that artifice might've not been a problem if the era didn't take itself so seriously.
Colin Baker.
I've said before that there was a bit too much fan-influence on the show by this point, particularly from Ian Levine. But Colin was actually a fan with a few good ideas for returning the Doctor to his darker roots (though that probably wouldn't have even been necessary if we'd skipped Davison altogether).
I thought Twin Dilemma did the idea stupidly and obnoxiously, but a toned-down, more careful version of it might've worked. And I have to say by the end of Season 22 I was becoming genuinely intrigued by this Doctor and where they were going with it.
Sadly Grade put paid to that.
McCoy.
I feel they handled his character unbelievably clumsily in Season 24. His badass moments felt unconvincing and only seemed to work against the villains by children's pantomime logic. There also felt very little that was intelligent or alien about his Doctor that season.
By Season 25 he was becoming a formiddable figure again, and it felt like we were getting an overdue correction at last of the Davison era. Finally the Doctor felt like a maverick again, who was actually written by maverick writers.
That said it is enormously hard to ignore his atrocious beginnings, or for that matter, to feel like the rest of his era is manically over-praised in over-compensation. I'll just say I'm glad we got what we did from his Doctor. I'm not a McCoy fan, but I suspect his era was one more season away from fully winning me over as one.
McGann.
He did not get a good start in the TV Movie. Even watching at the age of 14 I felt the weaknesses of his portrayal was that the whole thing was just a chase film, and he seemed to just spend it running away. There never seemed a point where he proved his credentials by standing his ground as the Doctor, he just seemed to get completely humiliated and it was a bit unpleasant to watch. (One of RTD's shrewder points was that the problem with the film was that this wasn't really a story where the Doctor arrives to save the day. If he'd not landed on Earth it wouldn't have been in danger in the first place).
Fortunately the audios have rectified a lot of this, utilized the clean slate of the movie, and have presented one of the most intelligent and formiddable Doctors in the franchise. It's like they took the best elements of Doctors Four and Seven and produced perfection.
Again though I fell a little sad we couldn't have some how skipped from Four to Eight.
I'll leave any discussion of New Who Doctors for another time, possibly.