Normally I don't restart a Pokemon title - I keep the same one indefinitely. However, for various reasons I've played through Emerald several times. Probably because it's my first Pokemon game, I can play through the title numerous times without really getting bored.
Most Zelda titles I'm happy playing through once or twice, then moving on to other things. However, I keep finding myself coming back to A Link to the Past, Skyward Sword, and especially Ocarina of Time and Majora's Mask. Seriously, I must've played through those last two at least a dozen times each. So good...
Metroid Prime 1 & 2. These two are just an absolute blast to play through, and I never get bored playing them, especially considering I've yet to collect all the powerups for either, and I keep finding new ways to sequence break.
Chrono Trigger. I absolutely love the combat system in this game, it's why it's my favorite JRPG (well that, and also its time-traveling mechanics and well-written story). Also, getting the different endings for beating Lavos at different times is really neat.