Wait, are we talking about Jak&Daxter?... I LOVE THE GAMES! right after my Kingdom Hearts my second favourite, hence.. why I choose Daxter to represent me.. hehehehe
Jak X... hmm, I had fun with that game, if you liked the desert driving potions in Jak 3 it's recommandable, but other than that of course not your typical Jak&Daxter game. This is why I didn't finish it either
And if I may answer your question what my favourite Jak&Daxter in the trilogy is... I would have to go with... hmm... either Jak 2:Renegade or Jak 3 ... let's just compare them, and I say what I liked about them
Jak 2:Renegade, the game was a huge step aside from the first game, with the futuristic setting of Heaven city, and the control of the Crimsons (Hope that are they called that in english, played the german version) and with that came that are other plot lines are more pushed in the focus of Jak. Who also became a better main charachter with the adding that he could speak now. With that came also many chars that you could love... and hate *cough*Razor*Cough* Which made the game bigger, and you really felt like you fought a rebellion against a system, which turned out to be... okay spoiler alert, so I am not going to write further
Jak 3 The game had an amazing precursor (hehehe punny) and it surely lifted itself as much as Jak 2 up. with a new area, new chars who were good, and then even to get to see older chars and how they changed in between the games was great too. And then just... Heaven city... so many things have changed that it felt like a new area already, and the storyline was so good... sooo good. Yeah I really liked Jak 3, and I would love to go deeper in it but don't want to spoil too much.
But I played Jak 3 more, even 100% it, so yeah... my vote for favourite trilogy game has to go to the third one :3