Another vote for the OM-1.
There are 4 versions:
1) the original M-1, which was pulled from the market when Leica made noises about the name. It is incredibly rare, sought by collectors, and way too expensive;
2) the original OM-1, which offered as an option the ability to be adapted for motor drive. Apparently, many owners opted to have the adaptation done, so shortly thereafter ...
2) the OM-1 MD, which included the motor drive adaptation, and finally
4) the OM-1n, which added a flash ready light indication in the viewfinder (with Olympus flashes) plus some other small refinements.
Some things to note:
1) all models took replaceable accessory hot shoe fittings (they have PC sockets as well). There are different models of the fitting, and they are not interchangeable. They tend to crack, if they are over-tightened, and there are therefor a lot of cracked olympus hot shoe fittings out there. The "shoe 4" model was the last, and was also used by the OM-2n. It can be hard to find in uncracked condition. I have three of them, they are all cracked, but they all work.
2) all models were designed to use the 1.35v mercury 625 batteries that you cannot get anymore (because of the mercury). There are a number of ways to deal with the battery situation - I have the type of adapters that Camtech sells in both my cameras, which cost a few dollars, but pay for themselves. You don't need the batteries, but it would seem a shame not to have the OM-1's meter available, because it works well;
3) the OM-1 is truly a system camera - you can pick up used a huge variety of accessories.
More information throughout the web, but particularly at: OM info