If I were to rate a neoprene or padded cordura holster as most comfortable, leather would be right behind it and kydex just after leather. Now add in the sweat factor, things can change a bit. Fabric and leather can retain moisture like a sponge when you start sweating. Kydex will not. Leather and fabric will give, kydex won't. I've had no issues with it against bare skin and typically have my t-shirt on the outside. My biggest issue with discomfort actually didn't involve the holster but the grip on the pistol. The OEM grip that came with the Sig was very aggressive. Once I swapped it out for a VZ carbon fiber grip the problem was solved. Take a look at the
Silent Fusion Thunder holster. It has a suede lining where it comes in contact with your body. It's supposed to be their most comfortable IWB.