Leica may make the best rangefinder on the market, but Nikon has to make the best SLR cameras hands down. The two go together like strawberries and cream.
I used to shoot Canon. EOS 1-V HS and then a 5D. I loved the 1.4/50 and 1.4/35L , but my god did I hate the ergonomics of the bodies. That control dial on the back almost drove me insane. The camera felt like it was designed by a bunch of engineers, not photographers. The Canon interface always felt like it was in the way. Their approach really is geared towards automation, instead of manual operation and it drove me nuts. That said I think I would have liked the original F-1.
Then on a whim I picked up a Nikon F3-P and boy did it bowl me over. It felt as natural as a Leica.
A few years later I came across a black paint 1959 Nikon F. Nikon really broke the mold when they made that camera. It's a pain to load, but I just love picking it up and shooting. There is something very special about the original F and in my book it may be the best SLR ever made.
Last year I bought a fully rebuilt F2. It lacks some of the charm and personality of the original F, but you will probably experience operator failure, before that camera gives in.
Then I got a D700. Aside from the M8 it was the only other digital I have encountered that felt like a film camera, except it used a sensor. The ergonomics are nearly perfect and the camera never gets in your way (except it's a little too big for my taste). It works just as well set to full auto, as to manual mode. The RGB matrix metering is the best I have ever seen. It's uncannily accurate and highly predictable.
Leica SLR bodies have been a mixed bag of goods for me. The SL is a great camera, but some how we never really fell in love. The R6.2 is close to perfect, but crippled by a very mushy shutter release. The R8/R9 is a joy to shoot and one of the best cameras I have ever used, but a little too bulky and they tend to suck expensive CR2 batteries dry at an alarming rate. But regardless I really like the R8/R9 and mine gets a lot of use. Oddly enough the much maligned R3 is one of my favorites. I wouldn't mind coming across a nice clean MOT example. It really is a shame about the R line, because the glass is second to none.
Aside from the R8 I primarily shoot a mixture of Zeiss ZF and classic Nikkors on my Nikons, but I get Leica envy every time I look at my Summicron-R negs....