Double quoted for importance.
I don't think FFV is the least important in the series. As Hironobu Sakaguchi put it, V and VI were the experiments that lead up to FFVII.
My vote would go to either FFIII or FFIX. Neither of them are important to understanding how the series evolved. FFIII brought jobs back to make them a staple of the series, but not entirely since there were White Wizards and Thieves and Dragoon characters in FFII. FFIX also because not as many people played it as the other games, and it didn't contribute anything going into the next generation. It took all the advances from FFVII and FFVIII and made a game in that style with a more traditional fantasy setting and a ton of references. It was damn good and arguably the best title in the series, but it's not important.
FFII was arguably the most important game in the series. It established that every game was going to be completely different.