Well, it's a good question and inevitably depends how much money you want to throw at it and how much space is available for it to occupy.
I'm sure you'll have plenty of suggestions. I could come up with about twenty off the top of my head but I'll limit myself to one. You MUST have an example of a Pentax Spotmatic!
Forget the arguments about who brought out the first SLR with TTL metering - the Spotty brought it to the masses - definitely the Ford Cortina of the camera world and, as such, deserving of a place in your collection!