It's too hard choosing one so I'll just throw out a few of my favorites.
Age of Empires 2
Counter Strike:Global Offensive
Half Life series
Total War Rome
Donkey kong 64
Jak and Daxter
Spyro Ripto's Rage
Awe there all taken, no sims 3 for me. Oh well guess ill try to catch the next humble bundle so i'm not searching for free keys on forums in the middle of the night
Halo 3
Halo:CE
Halo wars
halo odst
halo 2
halo reach
halo 4
Although its kind of a tie between Halo:CE and Halo 3. Halo 3 has a great multiplayer while Halo:CE has an amazing campaign. It's too hard to chose just one. XD
CS:go
Ocarina of time
And im also doing a 100% play through of Jak and Daxter since it's my favorite game ever. I've also been about half way through Morrowind for months now and I should probably get around to finishing it.