'Twas brillig, and the slithy toves did gyre and gimble in the wabe

I think one of the best game endings is in The Legend of Zelda Majora's Mask, mainly becuase after beating the game, you may think you're finished, but if you got enough of the items you get to come back with something really cool and powerful and screw around fighting all the old bosses.

In my opinion, the key to beating dark link is through mutual attack. Since he mimics your moves, you can both get hit, but he doesn't have 12 hearts and three bottled fairies to revive him.