It's Always Sunny in Philadelphia is one of my top 5 comedies ever, but it didn't get its famous darkness until Charlie Has Cancer and didn't get legendary until Frank showed up and Dee was just as fucked up as the guys in season 2. Season 1 had too much of the writers trying to make Dee crazy but still more sensible than the guys, but the pilot is truly awful. The jokes fall flat and the cast doesn't gel well. I started watching in the third season when I heard the hype and loved it, I might have stopped had I been there for the pilot.
The best pilot for a show that pooped the bed (Sunny joke) for me was Lost. That thing was tense. Fantastic revelation at the end with the great "Guys...where are we?"