Wii Sports
I've never been big on sports (except maybe Mega Man Soccer), so I wasn't expecting much out of this except some cheap thrills as an apartment activity. I wouldn't say I was wrong, but I totally underestimated Wii Sports. I might argue that it's the best party game since Smash Bros. To contextualize this, I played Wii Sports with my three roommates, so we had a full team so to speak.
The games themselves are something of a mixed bag. Tennis is probably the most fun for partying situations: extremely easy to play, and way too entertaining to watch your friends flail around with a racket. Baseball, none of us have figured out how to bat yet, so it's hard to enjoy. Bowling is pretty cool and easy to play, but not as exciting as a time-based game. Golf is pretty fun by design; we're having some issues with club-swinging (mostly putting), but that's probably a learning curve situation. Boxing is actually pretty exhausting since it involves so much arm movement, which has its own pros and cons.
Like I said, partying with Wii Sports is way fun. There's more to it too, like a Training mode that allows you (and optionally your pals) to play mini-games related to the five core games and hone your skills, and a Fitness program (for the solo player) that's like Brain Age but without the "brain" part. I haven't explored these much so far, but I may delve further into them at some point.
Progress: Tried out all 5 games, haven't done much else