It's a series of platformer games, and Jak II has the best platforming, so that's my favourite.
Then the original, then X, then 3, which basically did what II and X did but worse.
Jak X was probably one of the best racing games on PS2, and definitely the best 'kart racer' game on there in my opinion.
I feel like the difficulty of Jak II is always over exaggerated though. I agree that some levels, like the Baron's tower one, could use more checkpoints, but the only really hard missions in the story are the race with Errol through the city streets, the invisible metal heads in the forest, the Baron's tower one due to the awkward jetboard sections, and the rescue friends mission has a few tricky sections. I'm probably forgetting a few, but they're the only ones that jump out at me for edging on unfairly difficult.