Chrono Trigger. Tough choice, but had to go with Chrono.
Call of Duty 4 is a great game though. They finally updated the setting to modern warfare and gave the game a real online mode, both of which upgraded the game highly. Call of Duty has always been a bit overrated though, there are several better FPS out there (Hello! Goldeneye! Halo!).
Chrono was proof though that Square could do more then just Final Fantasy games. I wish the series had gotten more love, because Trigger was truly great. One of the best RPG of it's time, and a game that I can vouch for in saying it still holds up very well. Whereas I was tired of COD4 within a month or so.