I often see judgement about Shatner's portrayal of Captain Kirk that fails to consider the context. It is criticized not only by comparisons to the production standards of today (directing, writing, etc), and maturity of the Star Trek franchise, but also by current social values. That does Captain Kirk and TOS a grave injustice. But so be it... there are plenty of people who appreciate TOS for what it was. I liked earlier portrayals of Captain Kirk, and try to forgive the mess of most Season 3 episodes.
Captain Jean-Luc Picard has his moments, ranging from terrible to terrific. He had a "younger" persona about him in the first two seasons, then shifted into a more mature attitude going forward. I wish Stewart had been able to blend up the character a bit more. He became frequently very stiff and stodgy towards the end (although there are some latter series episodes where he is more lively). For TNG, I liked the dynamic of having the XO more active in command, with the captain in a more supervisory position. But of course, it was still great to see the captain go on away missions.
Benjamin Sisko was the most personable of the captains. If I had to work for any of them, it would be him. I just wish DS9 didn't have so much "soap opera" quality to it. The first few seasons were rather dull for me... it was such an important move to bring in the Defiant. That opened up things much more. And then from there, it got much better. There was noticeably less "character lingering" going on (something that happened far too often in earlier episodes).
I never really liked Captain Janeway in Voyager. She was great in some episodes, but in many... yeah "psychotic" is a good way of putting it. It was sometimes very difficult to believe her character. Her inconsistent attitude about the prime directive was unnerving. Voyager conjured up some really fascinating and enjoyable episodes, but it definitely takes a back seat to other series of the franchise.
I think Scott Bakula wasn't a good choice for captain of Enterprise. At first. The beginning of the series should have been much better than it was. The character dynamics and interpersonal chemistry between the characters were just not there. More than just the writing... In season 3, I think Bakula was finally coming around. He grew into the role. If he could have started with his season 3 Archer personification in Season 1, I'd have a stronger appreciation for Enterprise.