hmmm...favorite game...kind of a hard decision actually. Too broad of a range. If you're talking PC, then Half-Life (Counter-Strike is cool but I like HL better). Can't wait for HL2 btw!
As for PS2...hmmm...Baldur's Gate: Dark Alliance would be my choice. Beautiful graphics, great gameplay, and tons upon tons of baddies to smash, bash, and torch! And Dreamcast would be Samba de Amigo or Soul Caliber.