I think it was the M9 that really first caught my attention, about a decade ago. Buying into the Leica M system was not affordable, at the time, but I maintained an interest. I followed Leica, seeing the advent of the M Type 240. After years of watching the Leica Type 240 evolve into the 246 and 262, and then the M10 being introduced in early 2017, I finally bought a new M10, in April 2018. I then added a pre-owned Monochrom Type 246 in late 2018. In hindsight, I wish I had bought a nice new Monochrom 246, sooner. One of the reasons I had waited until I could get an M10, was its high-ISO performance. Well, a little-known secret is that the Monochrom 246 does quite well in low light, at high ISO, being at least as good as the M10, if not a bit better, in realistic conditions. Plus, monochrome shooting is more enjoyable than I had thought it would be.
I do not think I could pick a favorite, between the M10 and the Monochrom 246. Each does different things well. Some ultra-wide-angle lenses produce color shifts when shooting color images, so my Zeiss 21mm f/4.5 Biogon C, a truly wonderful lens, is only used on my Monochrom. I can see wide-angle frame lines better, with the M10, as I wear eyeglasses. The M10 is nicely slimmer, but the Monochrom’s bulk is not a burden, and the larger battery size means that the Monochrom can usually shoot all day.
I still have my Canon and Nikon gear. Adding Leica was a post-retirement, mid-life crisis sort of thing. I wanted to shoot more artistic images, so it seemed logical to use a camera and lens that I view as, themselves, works of art. As was the case when I added Nikon as my second system, it was the lenses that drove my desire to add Leica. A significant difference is that I could use many Nikkors on Canon DSLRs, with a Novoflex EOS/NIK adapter, whereas I had to add a camera body to be able to use a Summilux-M 50mm ASPH, my favorite Leica M lens.
Canon 5Ds R/7D2/7D/5D/40D/1D2N; Nikon F6/D3s/D700/FM3A/Coolpix A. Lens selection undergoing changes; some favorites: Zeiss 2/135 APO Sonnar, Canon 100mm f/2.8L Macro IS and 135L, Nikkor 14-24/2.8G and 200/2 VR.