FFII was torture for me, personally. I really didn't enjoy the gameplay and the story, well, I can't remember much if any of what story there was and I think that says it all, really. I only played it in the past year or so.

FFVI I'm playing right now, but if you've already played it a million times then I would opt against playing it again. You may as well experience something new, even if it's just the last 10% of a game.

FFVIII would be my #1 suggestion (and I will throw my vote away) purely because it has a lot of good gameplay and it's extremely easy to finish. You don't have to enjoy the story or the characters - just focus on the gameplay. Make a murderous killing machine out of your characters, come up with funky junction combinations, limit your menu to commands that are weird if you like to do that sort of thing, play lots of Triple Triad... whatever you like, really. If you just want to finish the game, get to the last dungeon and then turn the Laguna card into 100 Heroes and walk through to the end without even thinking. It's the easiest of the series to complete.

FFXII was a great game and if you haven't completed it at least twice I'd say it's worth another playthrough. You might want to play a third time for a more 'complete' game, but I found that leveling to the point that you can have a complete game is very tedious when you don't have random battles and experience can come slow. It's my #2 choice of the options you listed, though.

In short...
FFVIII - You may not like the game but hopefully you can enjoy the gameplay and finish another of the series.
FFXII - A great game worth at least two playthroughs.
FFII - I actually don't like the game but it goes ahead of VI purely because of how often you've played VI.
FFVI - If you've already played it 'a million times' then there is no need to play it again ahead of other games you've not played as much / haven't finished.