"Best" is a subjective term, to say the least. But I can share with you what info I know, and what I have experienced with my NS-10's.
I was always told to make sure the NS-10's had PLENTY of power... generally any amp that supplied as much or more wattage than the speakers could handle.
I used an inexpensive Alesis RA-100 power amp on my NS-10's for years, and although I was used to the way they sounded, they really did sound awful in comparison to other studio's NS-10's.
I ultimately landed on an older McIntosh 2100 solid state power amp, that an engineer friend was selling. The difference in sound was amazing. So much so, that my NS-10's now get used for 70 % or more of my mix monitoring. They sound great. And how many times do you hear people say THAT about NS-10's?
Another amp that I tried on the NS-10's that actually sounded VERY good, was an ART SL-A2 power amp. I was skeptical, because, well, it's ART for heaven's sake, but it was recommended to me, so I tried it, and sure enough, it made the NS-10's come alive as well.
Ultimately though, I'd see if you can find an older McIntosh power amp, in good condition. These are the amps the engineers here in Nashville seem to sing the praises for, when paired with NS-10's, and I can attest to that now.