Belden 5000UE 12/2awg, many strands, flexible, substantial, cheap enough. In wall rated. My bulk cable choice for sure.
I have Belden 5T00UP, 10/2awg for my hi-fi, it is overkill, but still cheap. In wall rated. Measured well in audioholics tests.
Then there's monoprice, a cheap alternative with negligible performance difference at average speaker run length. Good value.
14/2awg is the min size I would recommend for any critical audio installs, though 16/2 is often used for whole house\building structured audio installs with dependable results.
I took home leftover 18/4 sol securex from a fire alarm install job and paralleled the conductors inside of the cable. This yielded somewhere around a 15/2awg size cable. At runs in my HT of ~40' I can't tell the difference between the 10awg conductors. Saved me some $$.
Speaker wire is easy to get carried away on, but remember, where you put your speakers and your room acoustics will affect the sound 5000% more than adding even a dollar to how much you spend on speaker wire.
Just get 12 or 14 awg and focus on the next battle.