My suggestions, in no particular order:
Sly Cooper series (PS2, 3 games)
Ape Escape series (1 game for the PS1, 2 for PS2)
Jak and Daxter: The Precursor Legacy (PS2)
The 3 original Spyro games (PS1. If you see a Spyro game for the PS2, steer WAY clear!)
Steambot Chronicles (PS2)
Psychonauts (PS2)
Final Fantasy VI - IX, especially IX (PS1)
Okami (PS2)
And I must mention, as I always do, the original Grandia (PS1). It tends to crash sometimes when played on a PS2, but if you can live with this, you're in for a treat you'll never forget!
Sly Cooper series (PS2, 3 games)
Ape Escape series (1 game for the PS1, 2 for PS2)
Jak and Daxter: The Precursor Legacy (PS2)
The 3 original Spyro games (PS1. If you see a Spyro game for the PS2, steer WAY clear!)
Steambot Chronicles (PS2)
Psychonauts (PS2)
Final Fantasy VI - IX, especially IX (PS1)
Okami (PS2)
And I must mention, as I always do, the original Grandia (PS1). It tends to crash sometimes when played on a PS2, but if you can live with this, you're in for a treat you'll never forget!