Depends what you're into, if you're into RPGs check out FF X, FF X-2 and FFXII. Some must play ps2 games that aren't RPGs are MGS 2, MGS 3, MGS 4 (MGS = Metal Gear Solid), God of War and GoW 2. For some light-hearted action games the Jak series and Ratchet and Clank series are lots of fun.