He and Penny are the foils, but they are the two weakest actors IMO - but since Sheldon is so strong, they “work” by default I guess.
Raj has great expressions, and Wolowicz has impeccable timing and is a talented mimick and pretty good physical comedian. And Amy FF is awesome too. It works, but there are those that make it work and those that tag along. They’re all getting rich off it, that’s for sure .