Sigh....
I own them all.
SKS - ~$200 (Romanian, Chinese) Cheapest fun semi, 3-4MOA, 7.62x39, 10 round max capacity, no detachable mags (I dont want to get into all the legal issues of adding detachable jammomatics here)
AK - ~$350 (SAR-1 from aimsurplus) Medium cost, 3-5 MOA, 7.62x39 or .223 available today, 30 round mags, mags are cheap ($7), super reliable, easy to take care of.
AR - ~$750 (Bushmaster shorty) Higher cost, 1-2MOA, .223, 20/30 round mags, mags cost $15-$25, very reliable, light fun gun.
I started with the SKS. I loved it, but soon, I found the urge to own an AR, and then an AK. If you shoot a lot, you will spend more money on ammo than you will on the rifle in a short amount of time.
If I could only have one, it would be the Bushmaster, just because I like AR style so much. However, they are all a lot of fun. You can't go wrong if you want to start cheap. Just buy the SKS, then if you decide you want to upgrade, sell it. You'll get your money out of it and be wiser as well.