I know what you mean about Thief, though I only got into them recently. I'm not sure if I have a favourite, though I have a lot of nostalgia from games. Me and my friends were all very nerdy and so a lot of multiplayer games hold places in my heart. But one which I especially love is Jedi Knight Academy, simply because no other game have I ever found just so easy to pick back up and lose myself for a few hours in.