Out of the franchise of all the games it has to be GTA. I think GTA3 was better than Son's Of Liberty, Vice City is better than Snake Eater, and San Andreas is better than Substance.
Snake Eater is an awesome game though. Vice City was a complete milestone and was hit with millions of people around the world. It picked up loads of gaming awards and is still a big game today. Snake Eater is also a new milestone. Eating animals and they wide array of things you can do makes it unique.
I still havent played a proper game of Snake Eater yet, But I will do soon.