I think Emily Short's list of recommended IF (http://emshort.wordpress.com/how-to-play/reading-if/) is very good since it breaks up games by category. Nick Monfort also has some good recommendations (http://nickm.com/if/rec.html).
Personally, if you're looking for the old time feel, I would recommend:
Anchorhead (big, Cthulhoid, genuinely chilling in places): http://ifdb.tads.org/viewgame?id=op0uw1gn1tjqmjt7
Christminster (puzzle-oriented, very tough): http://ifdb.tads.org/viewgame?id=fq26p07f48ckfror
Curses (puzzle-oriented, great setting, time travel): http://ifdb.tads.org/viewgame?id=plvzam05bmz3enh8
How to play these: http://www.ifwiki.org/index.php/FAQ#How_can_I_download_and_p...
Easy entry point: http://pr-if.org/play/
Easily one of the best detective games ever created, and also my favorite Infocom game. Time passes as the game goes by, things happen in the game world independent of the player, multiple-endings, its got it all.
Of course this is a MUD and not a standalone game.
"Where Roleplay and Tactics Collide" -- highly addictive, beware!
There's graphics and sound, but the interface is classic text adventure.