quite a tricky one this tbh...
GTA started out amazing, then they made so many versions/sequels in the top down view it got boring, the 3rd person view started which refreshed the whole game but again the most recent one was pap and repetative, nothing worth shouting about.
Agreed about FF going s**te after VII which is a shame, could do with a decent one soon, I didn't even bother with the newest one.
Football games are just the same thing over and over again just with slightly tweeked graphics and the odd extra to try and keep the interest so imo that doesn't really = a great game series
Street Fighter maybe? (aguably the same as what I said about football games though)
just had a quick google for the highest grossing video game series but not finding anything, would be interesting to see. aparently GH3 is the highest grossing game of all time but that's probably cos it cost so much to get it with the guitar etc lol