I have a 5kg bass, so I tried all the heavy straps. the Levys MSS2-4 is a popular strap, used by many bass players over the world.
pros: 100% natural leather, soft multi-layer construction, comfy and grippy, very durable
cons: very wide (I ended up with a narrower strap as this didn't fit my gig bag); the top layer stretches out in uneven way, so after some wear in, the top level has some visible bumps/blisters.