Final Fantasy XII if we're to look at entire cast including NPC's, otherwise it's a toss up between Final Fantasy VI, and IX. Both games have few non important characters, then again which game doesn't, overall a great cast.

As for Final Fantasy IV few of you keep mentioning, characters themselves weren't that bad, sadly though, a rather cheesy story made them look like a cheap low budget Mexican soap opera cast, all the amnesia, fake deaths, family issues...for God sake Pedro.