The orginal Red Faction was pretty great, interesting new weapons, and the whole ability to carve apart 90% of the landscape as you went was one of the most entertaining things ever. I remember a puzzle in the game where to release the mechanical hold on your vehicle, there was some elaborate jumping puzzle to manually do it...or you could C4 the locked control room and simply press the button. And the multiplayer was great, even if it was offline, with 4p splitscreen + bots. With the only bar on level destruction being one or two indestructable panels and a overall damage cap to the entire level, you could do wonders over the course of a deathmatch. I remember often having 2p splitscreen with no bots, just the two of us seeing how far outside of the level we could tunnel before the geomod stopped...
The sequel limited the geomod far far too severely in my eyes. Here's hoping the next sequel I've heard snippets of returns to its original glory.
And of course, the mandatory list, HL2, the Halo series (Which while fairly solid in my eyes, receive so much flak because console fanboys hailed them as super innovative, when a brief look at pc fps prior to them would quish that idea, and each sequel failed to bring much new content to the table, just more of the same really. At least in my opinion) And the rest of the list, doom, Timeplsitters series. etc~