FPS:
-Stalker is fun but be warned, the retail game is terrible and I advise using mods to make it playeble for a much better experience.
-Am I the only one that actually enjoyed playing Prey?
-Deus Ex is a must play if you haven't already.
-Far cry is a must play as well, it's cheap and fun.. if you ignore the last level
-Hl1 if you haven't played this yet, do it now and use the high texture pack. Expansions as well, you'll at least understand most of hl2 after that.
-Aliens versus predator & aliens versus predator 2. Great games, both of them.
-BF42 with desert combat mod, BFV with poe1 mod, BF2 using either poe2 or forgotten hope 2 mod. The last, fh2 is just released and pretty cool.
-Call of duty 1 sp is still a mustplay if you haven't already, the expansion ( which ALOT of peopel never played ) has terrific multiplayer that rivals and beats that of the newer cod3 in alot of ways imo. You can still find people online for this and there are mods that add ATguns and halftracks.
RTS:
-Age of empires gold & AOE2 are still awesome to this day. I never liked AOE3, don't buy the latest version of a game just because of the graphics when previous versions are better.
-Warcraft III+frozen throne, great campaign and online the custom maps are great fun!
-C&C, C&C red alert 1 are still the best of the series imo. I still really want to play C&C3 for the storyline but I thought the demo was kinda meh so no hurry atm.
-Warhammer 40k, dawn of war + expansions are fun.
-Company of heroes looks great, arcady gameplay though. I suggest you try it at least.
-Settlers III or IV, still incredibly fun to this day and doesn't look half bad. Slower building game if that's your thing.
-CivIV + BTS expansion, addicted to this atm. Turn based build game, incredibly fun to play with a friend.