one hell of a (something or someone)

1. An exceptionally good, exciting, or interesting thing or person. Marty sure is one hell of a guy, always willing to lend his friends a hand! Two cars for the price of one? That's one hell of a deal! It started out slow, but this turned into one hell of an exciting movie!

2. An exceptionally bad or difficult thing, activity, or person. This is one hell of a math assignment—I can't understand the first thing about it! Boy, that is one hell of a haircut. You're going to go out in public like that?

a/one hell of a...

(spoken, slang) somebody/something that is very bad, good, unusual, impressive, etc: We had a hell of a good time at the night club. ♢ I had one hell of a hangover the next morning.

This is sometimes written as ‘a/one helluva’.