I like the straps from Contax cameras. After experience with the G1/G2 cameras I bought a few extra straps from KEH. They are an elegant grey leather with non-slip suede surface on the inner side, and then each end has a length of narrow nylon material. There are also wider ones that I think were for the Contax SLRs. Very nice straps at very modest cost.
I was really disappointed with the strap that came with a newer Leica M, shiny and slippery on the inner surface... slides off my shoulder too easily.