I guess I'm in the minority as mainly a multiplayer gamer
#1 has to be Counter-Strike: Global Offensive for me, changed the way I play and think of games, and strengthened and formed many friendships. It's so rewarding and perfectly paced.
Overwatch is also an excellent multiplayer game, where you really have to play as a team to succeed. But I haven't played it in quite a while as there's always an irritating hero in the meta. :P
Then as the only singleplayer game, Rimworld generates amazing stories just organically. Your base practically never ends up the way you originally planned.