Dungeon Keeper would be a fantastic choice. The whole idea of playing the ‘bad guy’ rather than the hero is brilliant. It mixes simulation elements (managing a lair) with real-time combat more akin to RTS games. The sound is moody and mischievous. The humour is great. And it even features a first person mode in which you possess your minions. It would make a great subject for discussion and it holds up very well today.
I too think Dungeon Keeper is very easy to get into even today. Both this and Theme Hospital, released in 1997, had easily understood mechanics and thanks to tutorials (!) during the first couple of levels or whenever a new feature was introduced, I feel like they are very approachable.