I'm sorry, but the definition of famous is to be recognized by many people. Don LaFontaine isn't famous. Sure, everyone has heard him, but nobody recognizes him. Who could pick him out of a line up or even identify his voice among all the copycats?
And, if asked who has defined the Joker, almost everyone would say Heath Ledger.
Like I said, sure Hamill might be employed, but as far as being a star of one of the biggest movies of all time, he hasn't done anything near comparison. Ford, on the other hand, had a huge career with a ton of other good movies. I'm not saying Hamill isn't talented nor that his work as the Joker (or in Metalacolpyse, Johnny Bravo, or anything else) but that doesn't make him famous, especially compared to Luke Skywalker.