Most good platfomers.
I'm talking the original 'Spyro the Dragon' and 'Crash Bandicoot' trilogies for PSOne.
Same for the 'Jak'-series and the Racthet & Clank'-series
And the Rayman games that were platformers.
They are fun, pretty lenghty and always make you come back for more. Therefor, getting 100% is a really enjoyable thing to do. Main reason is that there aren't many annoying/boring things you have to do to achieve that 100%.
And there are always rewards: secret levels, unlocking artwork, new weaponry, cutscene galleries, etc.
And they don't take up as much time as the average RPG (which I enjoy as well, mind you!)
Mentioning this, here are two jRPG's that are totally worth playing out 100% without having to grind or do annoying stuff:
Shadow Hearts
Shadow Hearts Covenant