Yeah, Friends is definitely one of the shows that I can rewatch for a hundred times and still end up laughing. It used to get replays here every year and I often turned it on regardless of how many times I have seen it. I wish they made more sitcoms like Friends were - I don't really like the newer ones because the humor feels so forced and it is too predictable.
My all time favorite series that I can rewatch and I have done so, is Breaking Bad. I have the DVD's of all the seasons. It is hands down, such an incredible series that pushed the boundaries and creativity. It set the limit for TV series really high. However, I was left quite sad when I saw the ending (won't spoil it). I just wanted Walter to be better off
A series that has everything in it - action, tons of good humor, heavy topics like cancer, drug smuggling, shooting, ... just overall amazing.