They both are good for their own reasons.
Heathcliff was voiced by the legendary Mel Blanc, wasn't as lazy and was much more of a bully (seriously, remember that big dog? He was afraid of Heathcliff).
Garfield was voiced by the semi legend Lorenzo Music, was pure lazy, prideful, arrogant and a big pig. The way he picked on Jon & Odie was great though.
The tie breaker? Heathcliff wasn't made into one, but TWO awful movies nor was replaced by someone who couldn't do the right voice at all.