Some might say the journey is the best part of any game. A satisfactory ending can put a nice full stop on a full and hearty sentence

I've found the endings to Baldur's Gate (and sequels) to be rewarding since the games are very engrossing, text-based and the nice little cutscenes do well to end what were epic battles. Most of the very end of the series was a set of epic then epic-er battles, and the ending wasn't just a cutscene, it showed you what all your party got up to as well (text form, but still good).
Call of Duty generally had great finale/end levels - even some which appeared to be the finale before and actual final level (oddly enough - since the games do levels "chronologically", talking about Call of Duty 2 here). Call of Duty 4 was quite an epic finish. Not played World at War or MW2 though.
While it was a little of a letdown after such a game, Grand Theft Auto: Vice City was a great buildup and finale, and ending I guess (not that it ended if you still had side missions and stuff you wanted to do). Likely the best in the series - I've not played GTA4 yet however.
Star Wars: Republic Commando, the only good thing to come out of the prequel crap, had a great set of 3 levels - each with its own very, very satisfying climax due to a good mixture of difficulty and setpieces. Well worth a play - it's reasonably short. The ending ended on a terribly pulled off cliffhanger however, annoyingly, which was a bit of a final sour taste.
(Mostly) story-only games-wise the endings to the Phoenix Wright games are pulled off well - celebrations and whatnot. Like I said though, it's the journey - the cases are much more the fun and satisfying bits

(it's more a text adventure really). The old-era point and clicks hold up well for endings - I've mainly played more humorous ones, but Day of the Tentacle really stands out as a great (and long!) ending.
Elite Beat Agents has a very, very satisfying and long ending. Man, it's like 2 back to back levels (well, you can go to the second once you finish the first, but they continue on). Great stuff, probably my favourite of all the Onderan and EBA games.
RTS wise, for the time Red Alert was quite satisfying in a slightly silly way. The levels are dead easy to me now, at the time it was a real accomplishment, with satisfying (and long for the time!) cutscenes. Newer ones, well, I like World in Conflict which really, really did some good work on characters, acting, cutscenes, and finale levels (give or take). Most RTS games are a bit rubbish for endings as such - Advance Wars, however much I love it, either have bastard-hard end levels with an ungratifyingly short ending few text things, or an easy end level, again ungratifyingly short ending.
Okay, plenty of games I've never been satisfied with though. A few looking around my room - Chrono Trigger went into boss borefest mode for the end, as do many JRPG's (leading to pre-boss grinding for an unsatisfying tiny ending, which Chrono Trigger exemplifies). RPG's which barely clear anything up, and don't even set up for a sequel also kinda suck - Neverwinter Nights, Mass Effect (although it did have some form of ending after a final boss at least, just not much - sequel setup!) and so on (Jade Empire from Bioware actually did it better, with at least some text about what teammates did after you did whatever it was you chose to do). Fallout 3 was highly disappointing (as was Oblivion...I see a trend!).
Talking about endings without the full experience is hard. Sometimes it is obvious an ending is terrible (and makes the game worse because of its presence since it lacks the quality, finish or fineness the game otherwise had), but it's hard picking out good endings from otherwise mediocre games. You don't usually find the best, most fun parts of a game are at the end unless it is done quite cleverly (and even Portal, to me, sadly didn't pull this off, although probably because it was so short more then anything).