For Major character: Radar.
Some of his naive lines were priceless:
"There are naked women taking showers with all their clothes off."
"I'll be going to jail in my puberty and won't be released until my adultery."
"Yes your Hebrewness."
"Do perform circumcision on Jewish Hebrews?"
For Minor character: Col. Flagg (who actually only appeared on 6 episodes)
"Don't play dumb, you're not as good at it as I am!"
"You are dumb... VERY DUMB! But you've met your match in me!"
"I have nothing to do with Intelligence!"
Flagg: Are you familiar with the Chinese-Malaysian water torture?
Radar: No sir.
Flagg: Good, because there's no such thing... yet.
And one more line.
Potter (to Flagg): Why are you dressed up like an Italian usher?
Flagg: Can you keep a military secret?
Potter: I think so.
Flagg: I'm disguised as Ling Chao, Chinese double-agent.
Potter: Funny, you don't look Chinese.
Flagg: Neither would Ling Chao if he were dressed like this.
"TransEastern! You'll feel like you've never left the ground because we treat you like dirt!" SNL Parady ad circa 1981