Well first of all u Need to be thinking about what ur aim is with m+.
Do u want just to gear up and have a bit of fun or do u want to go competitive?
If ur choice is competitive then u have to first of all think about the pros and cons from each spec.
For meeles I could recommend u the Sub rogue, DH, arms or the Monk.
For ranges I would say to go with the Frost mages since u have insane aoe potential and a lot of cc to help ur Group with (+heroism and buff). Other classes like warlocks and boomkins lack a bit of that great Utility but offer other aspects.
If ur choice is not orien