Quest for Glory series Space Quest 2-5 Soldier of Fortune 1-2 Fallout 1-2 Darksun 2 X-Com 1 Diablo 2 Doom 3 GTA series Monkey Island series The older ones just seem to offer more replayability...
Settlers III...no really. Age of Empires Got KotoR1 from Steam...awesome! KotoR 2 is not available however
I always play Command & Conquer Red Alert 2 & Yuris Revenge, both SP, Skirmish and MP. Allied Prism Tanks own, but there's nothing sweeter than sending an Air Armada of Kirovs. I also love Sonic 2, It was the only game i had on the Mega Drive, I didn't need anything else, i didn't want anything else. Lemmings, A classic, one which i found had been ported to a Mobile Phone, but i can't remember which OS i was using at the time, Think Windows Mobile 6/6.5. Chips Challenge, Ah those were the good old days, Played this on my very first PC, which i promptly broke.... somehow. Goldeneye, Nuff Said? Think there's a few more, but meh. Sam
Total Annihilation with all expansions and patches. There's just something brilliant about building an army, then hitting CTRL+A (All units) and Attack-Move the enemy base. My old machine used to cry like a big girl when the explosions started. The sheer firepower pouring out was just immense.
Monkey Island 2 - I've probably completed at least a dozen times! Dragon Age Origins - Totally loved this game, the second I completed it I started a new game These days I barely have enough spare time to finish 1 game, let alone replay them!
I don't frequently play games but I must say I've replayed the Commandos series a few times, the soundtrack was great too. Also, Metal Gear Solid.
Something I've thought of a lot - the problem with SP games is the story is the same each time you play it. After a certain number of plays, it's just too predictable and repetitive. I think for a desert island type scenario you need something that's sandboxed. I'm thinking UT / X series / Elder Scrolls series. Probably X games - they're always different and there's LOADS to do. Make even Skyrim seem limited imho. Ahh the memories of my destroyer victory fleet that I used to 'pacify' sectors...
Demon's Souls/Dark Souls, played Demon's Souls more times over than any other game and will likely beat that with Dark Souls.
1) Red Alert 2, replayed it close to 10 times 2) Deus Ex 3) Total Annihilation 4) Sins of solar empire 5) SW KOTOR
The one i've replayed the most over the years is probably Jedi Knight II: Jedi Outcast. Oh and Age of Empires!!!
Morrowind deserves a mention for having endlessly varied and unique experiences on every play through (assuming you don't make exactly the same character and play it the same way, of course).
I still find my self returning to the Ultima series (not IX!), Ultima III on C64 and Ultima VII (BG & SI) on PC - most modern RPG's don't come close (Morrowind & Oblivion - modded being the exception). Last Ninja I & II, Gunship and Project Stealth Fighter on C64 Thief GT Legends Cannon Fodder 1 & 2 on Amiga