Bakemonogatari's Ending. Seriously, how has no one else said this one?
The US Gundam Wing Ending. Probably one of the few times ever that the modified US version of an opening or ending was infinitely superior to it's original Japanese counterpart. Plus it's got one of the most insanely awesome guitar riffs ever.
As starkly as it contrasted with all endings and openings from there forth, the original Shakugan no Shana ending was pretty awesome and definitely made you feel like you just finished watching an episode of ass-kicking awesome: