Super Smash Bros. Brawl, as always. Haven't stopped playing ever.
Asides from, been hooked on the Metroid series for the last two months. After years of knowing of the series, I recently decided to actually play through some damn games. I started Metroid Prime 3: Corruption a few years ago, but only got an hour through before I completely forgot about it. Since this obsession began in November, I've beaten Super Metroid, Metroid: Other M, Metroid Prime and Metroid Prime 2: Echoes. Right now, I'm playing through Metroid Prime 3: Corruption, Metroid II: Return of Samus and Metroid. The latter two are bugger hard.
Though this doesn't count, there's the Metroid Blast attraction as part of Nintendo Land. Best there is.