General consensus is that the MC21 offers the best compromise between ease of restiction and most modern design. The MC18 is fast straight out of the box, though an older design. The MC28 is the newest design with it's (gimmicky) smart card ignition and single sided swing arm, but a lot harder / more expensive to de-restrict.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball