nothing wrong with 5.56, it's cheap and components for reloading are numerous.
7.62x39 will be cheap to shoot after you buy the mags but as far as accuracy I couldn't say as my only experience has been through the AK platform. I've heard the AR's in this caliber didn't shoot too bad.
6.8 and 6.5 are more cost restrictive compared to the others, but reloading makes up for a lot of that cost.
In my humble opinion of course, 5.56 would be the way to go for now.