Don't know about "best" spin training aircraft, but I did my first spins on a Decathalon, just after my PPL exam, just to get over the fear of this mysterious "there-be-dragons-and-certain-death" stuff I'd heard about. It was the single best piece of training I ever did because it gave me complete confidence in flying aircraft. I mention this just to reaffirm your deisre do the training - very smart idea! This stuff about "spin avoidance" training is complete nonsense. You learn a lot more about flying, and your own confidence by spinning an airplane than you learn about spinning an airplane.
So, which aircraft? There are probably lots out there that are perfectly suited to it. The only one I would definitely stay away from is the Cessna 152 Aerobat for the reasons mentioned by FFF but also, because it is so stable, it takes such exagerated inputs to get it to spin at all that the entry alone often scares the willies out of first time students.
I've personally spun the Decathalon and Pitts (both S-1 and S-2) and they're both great fun, easy to initiate entry, and easy to get perfect recoveries. But you'll probably be alright in Chippies, Stampes, Bulldogs.
It does also depend on what kind of spin training you're looking for in the first instance - just to do a couple of erect ones to see what everyone talks about? Or are you interested in a full series of erect, inverted, flat, etc? For that kind of stuff, stick to the real aerobatic machines like a Pitts or Extra.
In any case, have a great time and let us know how you get on. Spinning is a blast and an immense confidence builder!
Pitts2112