If I just feel like playing through the game then I'll go with Fighter, Monk, Red Mage, White Mage. This party has everything you need. It's an incredibly fast and easy way to play through the game.

If I'm feeling adventurous I'll change it up. In the line-up I mentioned I replaced the Monk with a Thief once and it really sucked.

Fighter, Fighter, Red Mage, White Mage is interesting.

Pretty much I don't use the Black Mage or the Thief because I find them useless. A White Mage is the only character I consider mandatory but it could be interesting to try the game without one.