Playing Left4Dead2 versus with 8 friends, running my own custom ‘Random’ mod. That game was so great to play matches in with the right people - and very fun to code sourcemod plugins for as well.

Estradasphere and Ween pop to mind

Left 4 Dead 2 (multiplayer)

CloneHero/GuitarHero

Quake 3 Arena (or 2, or DooM for nostalgia points)

Honourable mentions: Disco Elysium, Nethack, Portal.

In Dutch: ‘graftak’ is a beautiful way to indicate an annoying/terrible person. Literally: ‘grave-branch’, which sounds nowhere near as satisfying.