It really depends on the nature of your particular Chaos as to whether they are a beginner or expert army. If you stick with Slannesh or Nurgle, then you'll have a more 'difficult' army to use. Khorne is pretty straight up, but can still be hard to win with, while most Tzeentchi builds are pretty easy, provided your dice roll well for you during the magic phase. Slannesh and Nurgle require a bit more thought. Also, Beasts of Chaos, when not using a Doombull, are the more difficult of the typical Chaos armies.
Orks and Goblins are a 'middle of the road' deal. While they can be difficult for some, most players find that the list offers a bit of everything and they get along fairly well with little practice.
Wood Elves and High Elves are somewhat the same as O&G. With just a few games you can get the feel for the army and make appropriate choices that have more of an affect on the game than your in-game tactical decisions.
Don't toss out Tomb Kings either... In most cases, while TK can win against most any opponent, it is very very unlikely that TK will score a massacre. The TK almost require that a certain percentage of the TK army perishes before the game is up. It's all in the nature of the army.