He did command the Defiant... when it got its ass handed to it by the Borg cube in the beginning of Star Trek: First Contact... But seriously, though, Worf is probably my favorite Trek character of all time. I always found that his struggle to balance his Klingon heritage with his Federation upbringing (along with 11 total seasons on television between TNG and DS9, plus 4 movies) made the character deep, nuanced, and compelling to watch (I also really like Klingons, and have to second the earlier comments about Gowron... Robert O'Reilly really reveled in that role. Martok was a really cool character as well after that business with the Federation/Klingon alliance breaking down got ironed out...). Worf also had command of the Defiant a few times on the series, and temporarily kept his quarters aboard her as well (until he and Jadzia started sharing quarters...).
An interesting anecdote I heard once about that opening scene of First Contact: During the premiere screening, Avery Brooks (Sisko) and Michael Dorn (Worf) were sitting next to each other. During the battle with the Borg, when the Defiant flies into the scene with Worf in command and NO OTHER DS9 REGULARS, Brooks reportedly leaned over and said to Dorn, "I don't remember giving you permission to take the Defiant..."
James Kirk. Captain of the Enterprise. In a side-by-side, none of the others come close.
Really though the comparison isn't fair. All the others are more well-rounded characters, complete with flaws. And they had the benefit of years and years of episodes where their personalities could be honed. Kirk was a capital-haich Hero whose only flaws have come with changing attitudes in the culture.
But what we got to see of the nuanced Kirk--it was there, you just had to watch--he was intriguing. I hope that Chris Pine will look to some more well-rounded influences as he continues assaying the role, because Jim Kirk was many things, but he was never Han Bloody Solo!