According to my brother*, most games are great to play when stoned.
But in terms of celebrating the spirit of 4:20, that's a tough call. Katamari is so fuckin' out there. but there could be better ones.
Try a Harvest Moon game, and tell yourself it's a hemp and cannabis farm.
*That's not one of those "It's not really me, I swear" things. I don't take those. I just prefer gaming as my vice.
EDIT: Forgot. I'm playing No More Heroes right now, and I guess that's a good trippy game. I'm having fun.
A flashy-first game is awesome when it comes out. A great-first game is awesome forever.
Plus, just for the hell of it: Kelly Brook at the 2008 BAFTAs