I could list some games, and I will, but I will say this. PS2 games are so cheap these days, you might as well buy it if you like the look of it.
With the exception of certain sought after titles (your Okamis, SOTCs and Persona, for example), most pre-owned PS2 games have a market value of around 50p to £1, £2 at most. While all these recommendations are good, worth trying and all, one of the coolest things about playing PS2 is being able to scour boot fairs, charity shops, etc, and coming home with something you've never heard of, spent 50p on, and turns out to be awesome. I guess that's just what I like.
I recommend:
Odin Sphere - beautiful high-res sprite art in a 2D side-scrolling brawler with interlocking stories all about Valkyries, developed by Atlus
Maximo - very hard 3D platformer remake of Ghosts 'n' Goblins
Forbidden Siren - uncanny valley facial animation second-sight horror game
Dragon Quest - Best. JRPG. Ever