These aren't necessarily great performances, but I think they are great moments in casting.
Sylvester Stallone as Machine Gun Joe Viturbo in Death Race 2000
"Oh yeah? I happen to hold the clam sauce!" or "You know, Myra, some people think you're cute. But me? I think you're one very large baked potato."
Steven Seagal as that commando guy in Executive Decision. When they have that whole montage of Seagal and his men getting ready to storm the airplane, then Seagal bites it right away. Hehe, wasn't expecting that.
Sean Connery as Richard the Lionheart in that stupid Costner Robin Hood movie. You didn't need an actor for this role, you needed an icon.
Tom Petty as Himself in that stupid Costner end-of-the-world movie The Postman. Same as above.
Dina Meyer as Batgirl in Birds of Prey. Sadly, she may have been the only reason to watch that awful show.