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.
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.