I'm a big fan of point-and-click adventure games, which in itself is a massively underrated genre, especially since 2000 when they fell out of the mainstream. By "point-and-click adventure games", I mean stuff like 'Monkey Island', 'Myst', 'King's Quest', etc., but those are all classics.
If I had to go through a quick list of underrated titles in my head, I'd think 'Beneath a Steel Sky', 'I Have No Mouth And I Must Scream', 'Sanitarium', 'Blade Runner', 'Simon the Sorcerer', 'Callahan's Crosstime Saloon', 'The Colonel's Bequest', 'Titanic: Adventure Out of Time', er...huh, I'll have to look over my CD-ROM collection to single out a few more.
I'm also a massive fan of FMV games. You know, that period from the mid-'90s when game developers thought that including live-action actors and cutscenes in video games was going to be the future of gaming? Unfortunately, coupled with the software limitations of the time and the generally low quality of production and acting, they fell by the wayside and are largely seen as the laughingstock of gaming.
However, the 'Tex Murphy' series, 'Gabriel Knight 2', 'Black Dahlia' and 'Realms of the Haunting' were all good enough to redeem its reputation.