I do believe that FFXII has replaced FFVI as my favorite FF. It is everything that I love about the FF series(Big bosses, sidequests, optional bosses, summons, gorgeous music, expansive cutscenes).

Empirically judging FFXII necessarily leads it to be judged based on opinions; and of course people are going to be nostalgically connected to other FF's so their judgments are going to be flavored by their past experiences with the series without focusing on FFXII objectively. This is especially true for people who have only played 2-3 FFs. You can't base a game's rank solely on one element of the game that you liked better in another game.
That said, I honestly have a hard time saying that this is the best FF ever. When I think of FF at its essence I think of Amano's art, Uematsu's music, and the kind of stories that Sakaguchi dreamed up. Uematsu's music defines +90% of the FF world. Amano's art influences, if not pervades, EVERY FF. Sakaguchi's storytelling is simplistic and is also part of every FF. Uematsu's music is the only thing missing from FFXII, but as I stated in another thread, I think that graphic abilities have grown to a point that Uematsu's music, which was absolutely essential in telling the stories of the early games, is no longer necessary for a FF to feel FF. So I suppose that something else has to be considered to set a FF apart as best. It must be FFXII's battle system. Detailed, complex monsters are an integral part of FF. Their detailed artistry and fighting styles meant that given the limits of graphical capacity in the past, fights had to be setup in random battles outside of real-time/space. But now all of that detail can be expressed realtime while still allowing for the gargantuan size of the games, and so the magic and fighting styles can be felt better than ever before, and I think that it will only get better from here. I see FFs in the future with the ability to express the complex job systems in realtime. Ninjas that move swiftly and silently, thieves whose dexterity can be exploited on different parts of the mobs' bodies, summoners who can summon realtime(ala FFXI but more spectacular), dancers and bards who hold concerts in realtime battle, et cetera. So although this active battle system is completely at odds with every FF before it, I think it conveys FF-essence more than ever before. [And honestly I have to give consideration to FFX-2 just because it was the guinea pig for the active system seen in FFXII. Too bad it was such a horrible game.]
Really the best FF ever is FFXI, but because it is online and the general FF population will never play it, I don't take it into consideration at this point. FFXII is the next best thing. Although it departs from the FF formula at a few stages, there is no better FF game, and will eventually allow for the creation of the ultimate best FF ever, maybe XIV or XV.
But really I don't care which FF is the best. I love them all. The greatness of the Final Fantasy series as a whole is what I care about, and that is something which nobody can argue.