From the start of my WoW multiboxing career I've always gone for the classes that are fun to play instead of the flavor of the month. Out of my groups so far the Shaman have the lowest priority. The enjoyment factor for each team has been something like:

1xPally 4xMages : Very very fun! Not really capable of killing hard bosses. Great for BG PvP and ganking. Perhaps the ultimate farming team.

1xPally 1xPriest 3x Mages : My first go at a tri-force style team, still boosting the priest through Outlands. Some heroics will be totally doable.

5xBoomkin : I love stealth so this is fast becoming my favorite team. This will probably be the team I take into the arenas.

1xDeathknight 1xPally 3x Warlock : Another take on the tri-force team. When both are at 80, all the "parts" will be swappable.

5xShaman : Just... not... feeling it. Plus they are still way too popular. I've only run across one other boxer who doesn't do shaman.