This is really a question only you can answer. We can suggest about 5 other brands and 20 other models but have no clue what your using it for, how much you have to spend, etc. For example, if money is no issue get a M4-6 Leica, that will be small. Cheap, M42 Pentax gear is probably the best for less. A Nikon FE2 would be a smart choice because it's lenses will work on a DSLR, but most old lenses work on mirrorless Sony's anyway. Always plan ahead and buy equipment that will be put to use, then get out and use it.
BTW...If shopping for a M42 Pentax, avoid models older than the Spotmatic F because batteries are no longer made for them (with expensive exceptions) and 75% of the time the light meters don't work. Nikon's and Minolta's seem to hold up with time better. Minolta XD-11 or the cheaper XG-9 were great bodies with great optics.