"I'm cooking up!" in a scottish accent. Problem is you can either never say it
or well you can say it way too much and have an issue.
"Two minutes Turkish!" in an english accent.
"I'm not trying to be funny." in an irish accent.
Honorable mention to: "ze Germans" in a english accent imitating a german accent, "I'm forever blowing bubbles" in a cockney accent, and "cheers love!" in any possible UK accent.