Flesh-Eater Courts - Relatively easy to get into as the start collecting box has most of what you need and there are are fairly few models. But...there are fairly few models, expect to paint a lot of the same thing. They do have at least one build that has been competitive for a long time, so a fairly powerful 'pick up and play' army.
Ossiarch Bonereapers - New models with decent variety and can be painted to look very cool (I think so anyway). Their strongest build has just been nerfed but they are still a very tanky elite army. You get to take Nagash!
Greenskinz - There is no old school Greeskinz army anymore, but orcs can be played as Ironjawz, Bonesplitterz or 'Big Waaagh'. They have a few competitive builds, but all rely on smashing and bashing, so you have to like that playstyle. Oh, and I hope you enjoy painting green!
Beasts of Chaos - In a bad place at the moment competitively and a lot of the models are older. One of those armies that you really have to love to get over their limitations, and paint all those goatmen!
Slaves to Darkness - They have some of the best new models in the game, but for some reason those beautiful new models have been given pretty ineffective rules. I would take them anyway - rule of cool wins for me. This is one of the most varied armies in terms of ally options and builds. You get to take Archaon!