Glenn Close as Mona Simpson. This kinda what I was hinting at with my comment. She played that role straight. It wasn't and "here is Glenn Close ;)". I didn't even know it was her for a looooong time.
All the baseball players in "Homer at the Bat", just because I like the episode overall.
Kinda in the same vein as Glenn Close, I'd say Dustin Hoffman as Mr. Bergstrom. Only that time I could tell it was him. XD Damn distinctive voice. But, still. He worked really well in the part and it is a great, heartfelt early episode.