Dark Forces II
I did a drunken playthrough (I highly recommend doing drunken gaming. Incredibly fun when you have a spare Friday night) of the first one back February or March or so, and I really loved it. I actually have a key for the second game from the humble bundle, but after reading what people have written about the steam version, I decided that it would be a good idea to buy the GOG version (which also comes with the expansion :)) rather than buying just the expansion on steam. Plus, I'm still haven't used Steam since the whole paid mods thing.
Chances are, tomorrow night, I'll hit up the liquor store, pour myself a brew, and start on Dark Forces II.