Any character that is/was comic relief for their respective cartoon is my favorite.. but here's a few just for reference (in no particular order)..
Cosmo from Fairly Odd Parents
Ron from Kim Possible
Cyborg from Teen Titans
Toad from the Super Mario Bros Super Show cartoons (whew! Long name!)
The entire cast of Pepper Ann from.. Pepper Ann
the entire cast of Reccess from Reccess (sp?)
the entire cast of Spongebob from. . . Spongebob -_-
.. and now some random quotes!
It's a planet? from Fairly Odd Parents (Timmy)
Cough up the beans! Eww, rephase please! from Kim Possible (Ron)
I won't be having any attitude from no boat! from Teen Titans (Cyborg)
* Toad can't think of anything from the SMBSS!
Hey.. hey! from Pepper Ann (when Nicky was in detention, that kid leaned over and tried to talk to her)
What do you want me to do with these leftover beans? Put it in the freezer. It'll hold till September from Reccess (the lunch-ladies)
Squidward was always there for us whenever it was convenient for him! from Spongebob (Band Geeks episode)