Be the cat’s whiskers/pyjamas
Definition 1 of 2
Pro Tip 1/3Informal Usage
Used in casual conversations, not in formal settings. 
I aced my exams; I'm the cat's whiskers!
Pro Tip 2/3Positive Emphasis
Expresses a high level of achievement or ability. 
He's the cat's whiskers at playing guitar!
Pro Tip 3/3British Origin
The phrase is stereotypically British English. 
You've heard? She's the cat's whiskers in London!