Don't know about best, but I really like my 12" 412 HD Progressive shocks compared to stock. at around $200 they also represent a great value - I also put the Prog fork springs. Significantly better ride by doing both. -
BTW, I am 230 lb - you would not want the Hd

 

I recently put a pair of used Harley FXDX (Dyna Sport) shocks on mine. Got them on eBay for a little over $100, and they're very nice. 5-way spring preload ramp and adjustible damping, 13-1/4" long (probably longer than you're looking for).