My recommendation would be:

sartharion

Drakes are jokes, the second drake when you have to go in the portal is perfect for two split teams, trash is easy, and the final fight itself will teach you a lot about moving and coordinating (although honestly I've done the fight simply healing through flame wall). Also the bag with 5 badges in it is nice.

Naxx is also a great learning environment, and where I started out doing raids with another 5 boxer.

Ulduar is pretty easy now. The first part is very simple if you have one boxer drive and the other just be the gunners. Just smash one and two together and then go slow. As for FL himself, sit everyone in the center, and just kite on the diagonal. XT-002 is actually JUST a tank and spank. Completely ignore the adds - you'll outdamage any possible healing during the heart phases. Just have the tank pick up the big robots. Simply put everyone 15 yards apart. Set up in two groups on either side of the door, with one healer per group. Razorscale is relatively painless - setup right in front of the harpoons, strafe out of fire, and just kill adds till he comes down, burn, repeat, burn, kill him. His trinket is still VERY good for tons of people, and for that he's worth killing weekly.

If you are at least badge geared, VOA is very easy (koralon is the hardest box fight in there imho - toravon is free loot, emalon is a tank and spank when you one tank him, and archavon when boxed is a tank and spank - you just sit all your character in one spot - he doesn't jump or do anything when everyone is in mleee range - AND he can drop a mount!)

Once you feel comforable with most of the movement and such onyxia is a nice challenge.

And now I'll stop rambling - those would be my starting suggestions.