Although I have no love for the OS my favorite games are DOS freeware and shareware games especially Dune 2, Bio Menace, Xargon, Tryplane Turmoil, Flashback, PoP 4D, Sky roads and ... OK, I'll stop. :) Try them with DosBox if you find the time.
For modern platforms I love Ascii Sector. It is the best Ascii game I have ever seen. You very quickly overlook the lack of fancy graphics because of how polished the game is.
For completely non-retro games I like the 3-D Star Wraith Games.
I may have missed it, but nobody mentioned Deus Ex? I lost a girlfriend over that game...
I'm glad to see lots of folks saying Daiblo II. When LOD came out, that game became my favourite for a solid year and a half. Honorable mentions for me include: Commander Keen, Test Drive 2 (1990ish), and Zork (1980ish).
most of the game i played was best...=)
the if wan a ranking list for wats the game i played from the most like and less like...
the 1st one would be the NFS series..=)
2nd goes to COD..=)
and go on..xD