Ernie Ball 4052 Poly Strap Brown

I've got loads of these. Simple, seatbelt style. The quality is nice for the money, not too stiff. They come in a wide range of colours (not all stocked by Thomann, sadly) so I've got different ones for each guitar/bass. The colours are also better than a lot of plain straps in that they tend to have a nice quality depth and sheen to them and are not weak or washed out like many cheaper products can be (the pictures don't really capture this).

They are also longer than the average strap which meant for me, the buckle ended up in the middle of my back rather than nearer to the hips. I was concerned about this originally but found I never actually noticed it (D'Addario also make very similar quality/price straps that are shorter if this is a problem). The price is also often lower than anywhere else including the big A. You can't go wrong.