Really? Don't get me wrong--I love em all. But I think that the 3rd Season had the best level of comfort among the cast. They were finally comfortable enough to come out of their shell and improvise without being nervous around each other, but not so savvy with their role that they 'acted a character,' like I noticed starting with Season 5--catch phrases, etc..
Season 3 they were still loose, but not nervous.
I know that doesn't make any sense... move on.