Well you need a black mage, a white mage (one will probably be a summoner) and two physical characters for a rounded party. Options for that are thief, knight, dragoon and some long range type of fighter seems like the choices.

For black mage my fave goes to Vivi. No one comes near, maybe Rydia gets second. Vivi is the shy character of the party while also fitting the one thoughtful about existence.
Choice: Vivi
Second: Rydia

For white mage I don't like many of the characters actually for it, but I do frequently pick a customizable character to be white mage. And there is a choice that works well for this. Yuffie has a healing limit and is a thief type, fitting two criteria while also fitting the hyper character of the team.
Choice: Yuffie
Second: Eiko

For the basic knight character I like Squall, he provides my party with an antisocial main character, also the character developed more then any other in the series based on seeing what he's thinking. I also like Cloud or Auron for this roll, but I don't like to bring Auron into this because he's dead.
Choice: Squall
Second: Cloud

My last character could be anyone of a few, Cid from FFVII, he fits ideas for a last character and is a Cid, Wakka's got cool long range and is a fun guy to add to the team. Laguna always deserved attention but I wouldn't want to throw him in an actual party with Squall because there relationship would be so easy to do badly. Other characters that would be cool are Barret, Tifa, Zell or Rinoa.

So I'd want to pick more then just 4 characters, they're the ones you choose to use but my full regiment would be:
Vivi- Black Mage
Yuffie- White Mage/Thief (ninja)
Squall- Knight/Hero
Cid- Dragoon
Out of Party
Barret- Gunner
Tifa- Monk/White Mage
Zell- Monk
Rinoa- Pinwheel/Sorceress/Heroine