ahh the famous Jak Vs. Ratchet arguement...
Personally I like Ratchet way more. The series has a great sense of humor and has very lovable characters.
Jak on the other hand just doesn't cut it for me. Jak 3 and Jak X were especially disappointing but Jak 2 was amazing.
Let me go though the games i've played.
R&C 1: I didn't really get into it, i didn't really like the story but it was great never the less. The lack of strafing was a big downer for me (I played R&C 2 First Before 1)
R&C 2: It had all the good bits of R&C 1 with a better story, graphics and weapons. The new sense of humor was really funny too.
R&C 3: One of my favorites in the series, I really enjoyed the mini-missions, Starship Phoenix and the Mutiplayer too. I've played through it numbers of times and enjoyed it every time.
Tools of Destruction: Fantastic, My favorite Game in the series, I was amazed by the graphics and gameplay. The Weapons and characters were all fantastic (Mr. Zukron is amazing)
A Crack in Time: Brilliant, I found it one of the most in-depth R&C games, The story was very good. The introduction of freerom in your ship was also a great idea.
Size Matters: I didn't like it that much, It was also too difficult. Fire Wrench was baddass though.
BUT WHATS THIS! A NEW GAME
Insomniac do an amazing job at the series, A lot better then Spyro the Dragon.