I can only narrow them down for you.
* Boomkin aren't nearly as good @ 19 because they don't yet have boomkin form or typhoon (either are essential against melee). They do have travel form, but lose out on the other big defensives that moonkin get a few levels later.
* Ele shamen aren't nearly as good @ 19 because they don't have glyph of casting lightning bolt while moving, chain lightning, lava burst, or frost shock. All are pretty essential to their mobility and DPS output. Tstorm is nice, but lacks when you don't have any DPS coming out. Add to that the absence of any decent heals (3.0 sec healing wave sucks) and it's easy to see why shaman are one of the least represented classes overall.
*Shadow priests are strictly worse than disc priests. Even as disc, you deal more damage than you do as shadow, and get the awesome healing to go with it.
* Warlocks are good because they get just about every necessary offensive spell they would use later on (minus Haunt) and get fear as a defensive. The only real downside is that their pets don't have any specials until like leve 26-28. That means no Sacrifice from VW. You also don't get Succy or Felhunter yet.
* Mages are also good because they are in a similar position to warlocks. The only real difference (besides flavor) is that their pet get's Freeze when locks pets don't get any specials.
Overall I would say that disc preist, mages, and warlocks are the top choices for caster.
My fav is Disc.