If you're prepared to go back a very very long time, I can still recommend a number of Infocom text-only titles. I don't think they've ever been bettered:
Deadline
Suspect
Witness
Then (still ancient) another game that was very well done and can still be played on Atari ST and Amiga emulators is 'Murder'.
It's an isometric view game set in old country mansions in a old-film thirties-style setting. The murders are randomly generated with several million possibilities. It takes the Cluedo idea but transforms it into something new that I haven't seen since.
Not particularly deep, but interesting and fun.