I know that there have been threads from several years ago on this subject. The strap lugs of the M5 require a flat strap without split rings.
One thread asked for the original Leica M5 strap.
I wouldn't recommend any old leather strap for a camera, even if it is made by (or for) Leica. That ancient leather strap is just going to break one day, and the camera is going to go crashing down.
Other people have recommended the Domke gripper:
Op tech EZ comfort strap:
My personal recommendation is Nikon's AN-48:
The Nikon exudes quality.
Unlike the Domke, it has the least number of geegaws and attachments onto the strap.
Unlike the Op-Tech, its shoulder pad isn't so springy that you fear the pad and strap are just going to rip apart if the going gets heavy.
Also, the Nikon strap has NO labeling on the strap itself, except for tiny "Nikon" logos cast into the plastic buckles.
Yes, you can pay a lot and get custom straps made by Luigi et al., but I think these ones here represent the most readily available and usable straps. I've bought all of them at one time or another.
I don't have any reason for recommending B&H except that they're the people that I've purchased from. I'm posting this just to summarize my experience for any future M5 buyers.