In the USA, the best NSR is the one you can get hold of with a title! Anything after that is less important.

The MC21 is the best mix of parts availability, ease of de-restricting and ownership. The MC18 is often quoted as faster, though is the oldest of the sensible options. The MC28 is the newest, but has complicated ways to access any decent power.

Most bits will interchange with the 21 though, so just about any kind of hybrid is possible.

Buy whatever you can afford, and worry about the power later.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball