Here are some games I own that I consider as hidden gems on the PS2:

-Psychonauts: Almost no one cared about this game, regardless of all of the critics raving on the game. It's a great platformer with awesome voice acting. Some of the item collecting is tedious, but it doesn't detract from this great game. It may be the most well known hidden gem, but it deserves to be one since it didn't sell too great.

-NHL Hitz 2002 (practically the whole series): Not too sure how much this game sold, but this a personal hidden gem to me. One of the most fun arcade hockey games out there! It's pretty much NFL Blitz in a hockey ring. This is a fantastic multiplayer game. I think there were only two follow ups to this; NHL Hitz 2003 and NHL Hitz Pro.

-Crash N' Burn: I really don't understand why critics hated this game so much. This is a pretty cool destruction derby game. The crashes are great, and the game plays great. The only issue is that it's one player only. But if you like destruction derby games and want something other than Flatout and Destruction Derby Arenas, this is a good alternative.