Any cleave team, or triple DPS.
And probably a team with a holy paladin because they're comparatively easy to play.
Basically, everything that has a relatively low skill ceiling is best for breaking 2k.
But in the end you'll have the most long-lasting success if you stick to the class(es) you like the most.
A top gamer is always able to reach tournament level ratings, no matter his class. There's even a really high-ranked Warrior/Hunter/RShaman team.
Sub-2k rated players are often way too focused on worrying about class/comp balance or T2 items. But in reality it's not a big deal. Your teamplay, coordination, overall awareness and personal skill matter so much more I can't stress this enough. Sub-2k players are usually bad in that regard and thus will often lose to well-coordinated burst or swaps. Identifying and countering these situations, as well as setting them up for your own team, is what will make you a much better player.
Sure, RLS is probably the most efficient comp right now, or Lock/shaman/X in general. But there are many other comps you can play to like 2600+ right now... so that's not an excuse at all. Yes you will have to put in a bit more effort, but it is entirely possible, and that's what matters.