Its Smash Bros. If you have a 3DS this is one of those required games haha. Casual or competitive, this game rocks all the boats, capsizes them then jumps to a new one and repeats it. I'm pretty casual and I main MegaMan, Palutena, Pit/Dark Pit, various Mii Fighters, Shulk, and Mario. I enjoy making Mii fighters the most and using the Customs as well. Creating fighters in fighting games is always a very fun and varied thing to do.
Either #1 or #2 on my favorite games of all time. Played during a particularly bad week in my life, I found solace in Gordon Freeman's adventure. Save for the final level (which kinda sucks), the level design and gameplay were unbelievable for its time. Totally changed FPS for the better.