Ah yes, Chomper's a cool character alright. I'll talk about my views on each of his incarnations:
II- Definitely one of the best aspects of this film. He was really well designed, and I'm not sure what, but his general build really displayed a sort of baby-like texture to his skin. Scaly, but soft at the same time, especially in the face. His voice was pretty cute too, even if he just spoke in coos, growls and occasional garbled words.
V- A bit more of a mixed bag. I kinda wish they had kept his design closer to his cuter II design, with the bigger eyes and fleshier design and so forth, but I guess they changed it to make him look like he'd grown up. He just looked so much cuter and expressive in his original form. But he was a pretty cool character in this one, and had some great lines (Case in point, "YOU CAN'T DO THAT TO MY MOMMY!"). The only thing that really let him down was his voice actor. The studio took a risk with actually hiring a little kid to voice him, and it turned out to be a mistake. The kid's acting was definitely hit and miss, especially in the middle. Though otherwise, he was pretty solid as a character.
TV series- I think this is my favourite version of him. he just brings a great new character to the series with his quirky personality, his carnivorous nature, his gross eating habits, it just all plays well. His gullibility and simple nature really works to bring about all sorts of interesting turns, like the classic scene of him completely losing it after one of his teeth falls out. His expressions, timing, actions, all perfect cartoon gold.