Canon EOS 3.
EF 70-200mm f/4L
The first one because whilst it was a masterpiece of a camera technologically (and still is), and in fact was good at knocking in tent pegs on one occassion (so it was robust), it was big and ungainly, and too much automation for me, I prefer simpler cameras. I replaced it with my OM2n, never looked back either -- I think my pictures have improved immensely since then. One good thing was for a year or so I only had an EF 50mm lens (my 24mm broke when some water dripping from the Westway (an elevated motorway in Central-West London) parapet got into the lens...) which made me learn to appreciate the beauty of the 50mm standard aspect.
The 70-200mm, same thing again, great lens, took a few good pictures with it, but I felt I right tit with it, far too big. It drew far too much attention (on me, if I was 6' and 16st maybe it'd blend in, as it happens I'm 5'6" and 10 1/2st) -- I really hated it. I only bought it because I got a bonus from work and the previous day I missed out on an 80-200mm f/2.8 (thank god I didn't get that -- it'd be even worse) and was not thinking. I didn't want either really. I just do not like big cameras and big lenses for my use. I prefer to be more discrete, more approachable, less woah big lens, and get sick of people asking me "So are you a pro?" which really bugs me big time, I'm just taking pictures.
Sadly when I finally got around to selling it at a pretty big profit, it was stolen from my car boot as I thought I'd take it with me to Milton Keynes just in case I found anything worth photographing with it there. As it happens, I very rarely find anything I want to photograph at >100mm and it wasn't used. The EOS 3 went shortly after.
I think my other big regret is shooting some of the MK project on Fuji Colour 100 -- horrid film in my experience. Everything looked like the green cast of nuclear fallout was smearing the scene, highlights burnt out, colours bleached. Nasty nasty stuff. I reshot most of what went wrong those couple of days. What is worse is that I have absolutely no idea why I tried it, I could have got Reala which I know is OK. I think I got drawn in by what I saw on Flickr but for some reason my experience of it was awful. I gave all that I had left of it away and have gone back to Kodak for all of my colour film (except Reala).
Vicky