I'll second the D4 as one of the most reliable and most used cameras that I currently have (it has over 300,000 actuations). The shutter gave up the ghost at about 300,000 a couple of years ago, but after having sent it to Nikon for an overhaul, it worked as good as new.
I actually have two other digitals that seem to be really reliable, though it's still relatively early days. The Nikon Z7 has been working very well and hasn't let me down (knock on wood), but it definitely has an appetite for batteries. That would be its only downside that I can see, but for all the commercial work I do with it, it's been great. Can't say enough good things about the Z lenses either, though some may not like their physical size (doesn't bother me). The other camera is the Hasselblad 907x - it's been very reliable and, though I don't put it through its paces quite as much as the Z7, it's extremely versatile and produces top-notch files.
Actually if I could add one more to the mix (though I no longer have it), it would be the Leica M-D 262 (the one without the screen). Never had any issues with it at all, and using that camera made me realize how unnecessary the screen can be (as well as how distracting it has the potential to be). Super-duper camera, and I've often thought of buying another. Definitely the best digital M I've ever owned.
