Definitely Grover. But I've noticed how Frank Oz has a tendency to ham up virtually all Muppet characters he performs, including on "Sesame Street." Grover has many hammy moments, but so does Cookie Monster, and Bert sometimes does as well. And then there's all those minor and one-off characters Frank Oz has performed on the show over the years where he'd ham it up and make the sketch even more fun to watch; my favorite is probably when he performed Pinocchio in that 1976 News Flash segment.
Maybe it's because his voice work isn't as versatile as other Muppet performers (I mean, look at how nearly all his female characters have that same Miss Piggy falsetto voice!), so he focuses more on the drama and acting aspect of performing a Muppet character. He sure does a great job with it!