Any of the Super Smash bros games. It probably goes without saying, but it's great for crowds, and even those who are not that into gaming can get into it. It's also easy to pick up on.
The game that I most enjoy playing with friends is any game in the Mario Kart series. Not that I don't enjoy playing it by myself, but it seems like my brothers and I had a lot of fun playing Mario Kart. Especially the battle mode.
I don't really have friends... Sometimes I play Cards Against Humanity with some of my online acquaintances... I used to play fighting games with my high school colleagues...