Best? Its a matter of personal opinion and difficult to say...but...
I have a friend who had the Piranha and Maranello in both direct drive and gear box, and he comment that he liked NR 21 better cause he felt the NR have a little bit better power. During race, this guy's boat could not catch up our CMB 21 LS and CMB Valvola (CMB version of MAC21) at straight even we race the same hull. Over here the Rossis are a rareity because local support on parts is non-existance, so in our case it is not "the best" engine here....
My 0.002 cent....