Favorite Season
It's hard to pick between the 1st three seasons. It's a toss up. But 2nd favorite would have to be season five. Yes, season 5! Yes, it didn't have a couple of the main characters and it changed a lot. And that's all the more reasons for it to be at the top. I can't see any other shows keeping the same standards if two of it's lead left. I can't see Kramer leaving Seinfeld, along with Elaine. Or Joey leaving Chandler, plus, someone like Gunther or Janice (a 3rd Friend is too big for Piers). Or Barnie leaving, with Lilly in HIMYM. They really explored the rest of the secondary characters.
I don't think season 4 was bad at all, either. If anything it was too short.
Season 1
Season 5
Season 2
Season 4
Season 3
Season 6