Depends on how much you want to spend on one really.
Up until relatively recently, I used an unpadded, 3" wide, plain leather strap. Sturdy, didn't slip, and more importantly, relatively cheap (about £15.00).
Been getting some pain in my shoulder for a few years, so finally decided to "bite the bullet", and spend a bit more money on a decent padded strap. Finally settled on the Mono GS1 (black), which is comfy, doesn't slip, and appears to be very sturdy. I got mine from Amazon UK, for £35.00 (still the cheapest in the UK for the Mono strap).
Looked at the Comfort Strapp & others, but the Mono was the right price at the time, and does the job I ask of it.