I happen to be watching Boston Legal which I have only seen once or twice before and not only are Candice Bergen and William Shatner in it, the guest star is Michael J Fox. Pretty good - three genuinely iconic TV performers.
Any other offers?
Well, there are shows whose entire casts are iconic: I Love Lucy, The Dick Van Dyck Show, The Mary Tyler Moore Show, Seinfeld, Friends, etc. (I don’t necessarily like all these, but they are iconic).