Ive all three of those rifles and some. Heres my thoughts..
AK-47 cheap, very cheaply made, inaccurate as they get.
Whatever you choose, you want Ballistic tip or soft point bullets intended for deer hunting.The AK-47 usually fires military full metal jackets, which makes not so lethal ice pick wound. Similar to the power of the 'ol 30/30. Of course it is reliable, most military type rifles are. Definitely better than throwing rocks.
AR-15 fires a .223 Remington bullet. Turbo .22... That's a "varmint bullet" for prairie dogs, not serious combat. The bullet is so small and light it has to depend on high speed to
fragment the round inside the target. (Yawn)
SKS very similar to AK-47, more accurate, minus pistol grips, and above all....the damn magazine is fixed. Its part of the gun, so no quick mag changes, and you must load it through the top, with the breach open. Better off with the AK.
What it really comes down to is the ammo they all fire.
For serious engagement of an enemy, even behind hard cover and want to kill outright, you want something chambered for .308 Winchester/ 7.62X51mm NATO. There are many hunting ballistic tipped and soft points out there in .308 as this is a very popular deer loading. In the 7.62NATO, the military equivelant of .308, There is also armor piercing, tracer, armor piercining incindiary, and the REAL "One shot, one kill" round, the M852 168gr Sierra Matchking BTHP. The .308 & 7.62NATO look identical but are NOT always interchangeable, although Ive fired hundreds of .308 soft points in my CETME. High quality commercial .308 runs fine in 7.62NATO rifles, IF the brass is THICK enough. There are many fine rifles in 7.62X51mm such as Springfield M-1A,
FN FAL, even the AR-10. My personal favorites are the HK-91/ CETME/ PTR-91 type rifles. Uber-reliable and more accurate than an AR, Go for the PTR91 or CETME as the HK haven't been imported since early '90s and are expensive.
Heres a little table for comparisson in velocity and energy
Cal weight of bullet velocity energy
223/5.56mm 62 3100fps 1336ft/lb
7.62X39mm 123 2300fps 1480ft/lb
.308/7.62X51mm 147 2800fps 2581ft/lb