While I like Mike Farrell, to me, Trapper was the original role.
Great guy but, of course, not really intended to be anything more than Alda's sidekick.
Same with Beej.
Both are good.
Here's how I like to state it:
MASH was best when Larry Linville was on it. That way, we get the best of both Trapper and Beej.
Not sayin' "Chas" wasn't any good. DOS was great, but Frank was one heck of a character.