My reccomendations would be any of the following;
Manual focus, manual exposure;
Nikon FM2, all mechanical, the only thing the batteries run is the lightmeter, you have to truly learn to take pictures with this one, and that knowledge will translate well into whatever camera you use in the future, plus, it’s a great camera to have as a backup body
Pentax K-1000, same as the FM2, all mechanical, just uses Pentax’s lens mount
Automatic exposure, manual focus
Nikon F3 (actually, any of the F series), the F series are Nikon’s proffesional series, the F3 is a very rugged, reliable camera, built to take the beating of a proffesional, and since the camera has been around since the 70’s, you should be able to get a decent used one at a good camera shop, it shoots in manual mode and Aperture Priority (you pick the aperture, camera picks the correct shutter speed)
Automatic exposure, autofocus
Nikon N6006, 8008, N90, or F4s
the first 3 are Nikon’s consumer AF cameras, the F4s is the Pro model, i have this one myself, i love it, it’s built like a tank (aircraft aluminum frame, body alone weighs 3 LB), it can shoot up to 5.7 frames per second, it’s an awesome, rugged beast, you could probably use it to bludgeon a competing photographer out of your way, and still use it to take pictures… ;), all the above cameras can shoot in Manual, Aperture Priority, Shutter Priority (you choose the shutter speed, camera chooses the aperture, SP is better for action shots, AP is better for landscapes and shots where you need control of the depth-of-field) and Program (camera does the thinking for you )
one thing to keep in mind about these old AF Nikons is that they’re based off Nikon’s old single-sensor AF, the newer Nikons have the newer multi-sensor AF
personally i’m partial to Nikon, primarily because the F-mount has remained largely unchanged since 1958, it’s been adapted to Autofocus, but i could still take an old, classic Nikkor made back in '58, slap it on my F4s and use it, i’d be limited to Aperture Priority or Manual, but the fact remains that it would still work
try putting a Canon FD mount lens on an EOS, or a Minolta MD lens on a Maxxum, it ain’t gonna work (at least not without an adapter)
if you truly wanted to learn photography the correct way, you’d get a FM2 or K-1000 and start from scratch, that’s the best way to learn, automation is for the lazy (says the guy who primarily shoots AP, and refuses to use Program )