Boss fight: FFVII, that Sephiroth fight was great.
Climax: Really hard to say as I can't think of many endings I was really satisfied with just how things went down. Perhaps FFVII. The world defeating the comet was alright and all that. Oh, or maybe Skyrim. That was a really cool way to end a game.
Post-climax scenes: FFVIII. While the Time Decompressoin cutscenes were nonsense (although great for emotional storytelling) in general, that credits post-ending stuff with them all celebrating and whatnot was spot on. More Final Fantasy games should do this kind of thing.