Personally, M4C has too many fanboys and guys posting from their mom's basement for me.
Purpose is the primary question like globemaster3 already said. Once you decide on purpose, then you have to prioritze budget, accuracy, durability, reliability, ergonomics and aesthetics.
ARs go from $300 .22LR carbines up through $4000 large caliber AR10/SR25 based rifles, so there are LOTS of choices.
With that said, for most people, it is usually a good idea to either buy a catalogged AR15 in .223/5.56 from one of the larger reputable manufacturers.