Community is without a doubt one of the funniest shows I've ever seen. It's amazingly wacky and the characters are so flawed and realistic. The cast is perfect. Every actor nails their role, and the writing is quick and smart. You'll initially laugh at the primary joke in every scene, but then as it sinks into your brain, you'll realize there were 3 more jokes hidden within it.
Now I will say that the last two seasons have a different flow to them due to some of the cast leaving, but they're not worse, they're just different. The series finale left me in tears. Just watch the show. It's only 6 seasons (and a movie).