What You Leave Behind - It's not perfect, but I think it's easily the most complex, engaging and powerful of the finales. IMHO it's a worthy conclusion to DS9 as a whole. My biggest complaint is the final Sisko / Dukat confrontation which is really underwhelming, I think. Other than that (and minor complaints aside) I really enjoy WYLB.
In terms of the other finales, I think All Good Things is also very good. It's a great way to end TNG IMHO. It's as satisfying a conclusion as I could imagine for that show given it's episodic nature.
Although Turnabout Intruder was TOS's last episode, I wouldn't really count it as a finale. As an episode (and as far as I remember it), it's decent.
Although I was neither a fan of VOY or ENT, I was curious about the finales. My thoughts: "Boy, if I WERE a fan, I'd be really pissed." Endgame just seems to stop without any emotional payoff at all. And These Are the Voyages is more like a TNG episode, and not a very good one at that IMHO.