If we look at it as most important being the best, I give it to FF1. Pretty hard to beat the one that started it all and it's a solid game overall.
If we look at it as most popular being the best, it goes to FF7. It's a good game but severely overrated imo. I've said a few times my opinion about it and that hasn't changed. Good game but hogs the spotlight from other great games that deserve it.
If we look at it as the best game in the series, it's arguably FF9. This was the pinnacle of what the franchise was meant to be and was considered to be a love letter to the fans.
FF10 was good and is what I generally tell people just getting into the games to start with. Pretty much everything about it is solid. (It also has my favorite version of Bahamut so it gets bonus points from me there).
As far as mascots go, idk about game specific ones but Black Mage, Moogle and Chocobo are probably the most iconic characters in the franchise.