1-What a riot!
2-You are a good sport.(being good sport)
What a riot!
Presuming we're not referring to an actual riot, this phrase refers to a raucous good time - a fabulous, hilarious party or outing or experience of some sort.
"You missed a great party last weekend! We all went to a karaoke bar, and Fred, Bill and Bob sang and danced to Beyonce's "Single Ladies" song. I laughed so hard I couldn't breathe. What a riot!"
You're a good sport.
As mentioned above, being a "good sport" can refer to losing a sporting competition with good grace. It can also refer to someone who maintains an upbeat attitude and sense of humor (rather than getting angry, crying, or vowing revenge) when he is the butt of a prank or joke.
"Roger was obviously embarrassed when the stripper we'd hired arrived at his birthday party and started taking her clothes off. But he was a good sport, he let her kiss him and rub up against him as she danced."