After my PS2 broke (I don't know what happened ): I reflected upon all 20 or 30 games that I have and realized that they're basically all shit I can live without, and so save $100 on a PS2 repair/replacement and trade in all my games. Really, the only ones that I'll miss are Shadow of the Colossus and Ratchet Deadlocked. It's unfortunate, but I really think that most of the games for that platform are soulless time sponges, which isn't really what I'm into.
But if you've already gotten one, then Shadow of the Colossus FOR SURE. One of the best games ever, imo. Also, the first Ratchet & Clank, and Deadlocked are both tonnes of fun (I never got into the 2 or so in between.) If you don't have a PS3 to get the collector's edition, I'd suggest the first God of War as well (because, to say it politely, 2 and 3 can suck my dick.)