The best dialectric is air. Actually, the best is vacuum, but that's a touch more tricky in practice. The problem is that with 'normal' cables in hifi use, it's effectively impossible to use air as a complete dialectric, as you need something to support the conductor and keep it in place. For low signal level use, you also need screening, and to be properly effective the screen needs to be relatively close to the conductor, so a physically solid dialectric is often the only sensible solution.
Nevertheless, the practical differences are so small here that I doubt the insulator (or lack thereof) is likely to have any real effect. IME, the actual wire construction (solid core, stranded or Litz) is the one thing that has an apparent and repeatable effect.