cavestory.org
Now I have two things to say:
1. There are hundreds of reviews of various games posted here, many of which are very thorough and descriptive. Use those.
2. If you would like to ask the community for suggestions, narrow down your selection so that people will suggest things you'll enjoy and be able to play, because I guarantee that 90% of the people on this forum either don't have the systems to play most of my favourite games, or wouldn't be interested in them. So if you want suggestions for some good games, ask something like, "Hey guys, I'm looking for good casual fighters for the PS2 or PSP. Any suggestions?" or something along those lines.
3. Warlords III: Darklords Rising (PC, TBS), The Chaos Engine: Soldiers of Fortune (Amiga500, ShMUp), BeatMania II DX series (JPS2, Rhythm), Monster Hunter Freedom Unite (PSP, Action/Adventure), Megaman Series (various platforms, 2-D platformer), Portal (XBox 360/PC, Interactive/Puzzle), Deus Ex Machina (PC, FPS), Warlords Battlecry 3 (PC, RTS), Civilization II (PC, TBS)
Those are all excellent games that I would recommend to any serious gamer looking for challenge, enjoyment, and replay value. I can only assume those are all factors that you consider highly relevant, since you didn't endeavor to further specify.