I really like uqm. It's one of those games that will appeal to those who like to acquire goodies in an RPG and see their character and their understanding of the game world increase in strength as they finish quests. It's a more long-term game than the ones you list, though.

Doom (PrBoom) is probably the easiest FPS to play because it doesn't require aiming (by design) or up and down looking.

Gweled is fun when you just have a few minutes, especially if you like puzzles.

One might say i'm biased because i ported and/or maintain them. Well, yeah...obviously i wouldn't spend time on them if i didn't think they were worth it!