What matters, in order: correct gauge for the run length, decent copper, solid terminations, and sensible routing. After those four, audible returns diminish very quickly, and I would rather you spent the difference on the system.
Gauge first, because it is the one audible cable mistake. A speaker cable has resistance, and resistance rises with length and falls with thickness. Undersize a long run, and the amplifier loses damping control over the woofer and drops a fraction of a decibel of level along the way. Thicker for longer runs, thinner is fine for short ones. Surrounds at the back of a 5 metre room on the same thin cable as the fronts is a common quiet loss.
Copper second. Plain oxygen-free copper is what you want; copper-clad aluminium sold as speaker wire is worse per gauge and should be avoided. You do not need exotic metallurgy.
Terminations third. A stray strand shorting across a terminal is how AVRs go into protection mode on movie night. Banana plugs or properly crimped spades, tightened, are worth the small cost simply because they stay put. Bare wire works if you are careful and check it once a year.
Routing fourth. Keep speaker runs away from mains cables where you can, and cross them at right angles where you cannot. Not because the cable will hum, mostly, but because good habits are cheap.
Beyond that? Measurement says a competent cable is a competent cable. Audibility follows measurement here more closely than the marketing suggests. We do not sell cable mythology, and if a cable's description contains more adjectives than specifications, that is your cue.
The part that is not mythology is in-wall. Cable going inside walls or conduit should be rated for it, and it has to be planned before the civil work, along with a spare pull for future channels. That is not audio science; that is the building, and the building does not accept returns.
If you tell us the run lengths and whether the walls are open, we will suggest gauge and termination and, where useful, sell you exactly as much cable as you need and no more.
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