Most modular LED flashlights

What are the most modular LED flashlights on the market? Something where the head, body, tailcap, etc. can all be interchanged with other models.

I'm going to say that the Surefire 6P and all the different variants based on that light as well as the different clones that have been produced over the years. The tail cap and bezel are threaded at 20TPI, which works out to 1.25M, which makes it easy for a company that has a metric lathe to cut threads for it. There are quite a few Asian manufacturers who make clones and variants based loosely on the 6P. In some cases they're downright rip-offs of that light! Even Surefire's larger lights like the M3 and M4 share the tail cap compatibility with the 6P. The newest 6PX tailcaps can be used on an old 6P and vice-versa. I can't think of a more copied design or one that has more support than this.

Peak's LEO series, including their Night Patrol, First Responder, and some of the Search and Rescue lights, all share common components that can be mixed and matched, with some limitations of course.

SF-6P by a significant margin. There are a LOT of upgrades and lego swaps you can do within this format.

Theres a lot of fun stuff you can do with D/C MAGs too.