FFVI is a hard one because... well, we never really got an official definitive version (IMHO anyway) so the fans had to step in. Not a bad thing at all, but there's a lot of efforts out there so the definition of "definitive" seems to have a lot of meanings with this one.
You have the Relocalization Project that has everything pretty well in order for making the game as it should be: big host of bugfixes, redone translation using the correct bases, uncensoring. Only thing that isn't there is the GBA extra content.
Then if you like the original SNES translation better, there's Ted Woolsey Uncensored. Which does what it says on the tin with a whole lot of work, but no bugfixes (some like it this way though, so I don't condemn it for that at all.)
If you want the extra GBA content like the final dungeons, translation, and official bugfixes, there's a whole bunch of patches out for that. All except for using two font patches at once works together it seems, so just use whatever suits your fancy.