Love this series. Great platformers.
The first one is still one of my favourite game worlds, especially the first village/beach areas. And the gameplay managed to have the same feel to it even after the big shift in the second game to guns and vehicles.
Jak II would have to be my favourite in the series (for some reason the 2nd game in a series usually is), just because it was simply great. It had elements of the first game and certain areas that were similar, and also it had the completely new aspect of the city environment, with the aforementioned guns and vehicles. It also expanded the sotry perfectly (unlike Jak 3. Seriously, the Precursers are Ottsels? I hated that), and the characters were portrayed perfectly (unlike Jak 3. Jak and Ashelin kiss? WTF happened to Kira and Torn?) and the graphical leap from the first game was amazing at the time.
The dark Jak element added some variety too, especially because it was never actually necessary to use it. The thing that cheapened it in Jak 3 (apart from the loss of giant dark Jak) was that it became a puzzle solving tool. Plus the environments in Jak 3 weren't quite as enjoyable. The city is in ruins and riot for the whole game, which got boring quickly. And the desert, while fun the vehicles, was also a litte dull. The giant robot at the end was good though.