Like others who've already chimed in, I doubt there is one single "Best" tire for any model of bike. It all boils down to how you use it, style of riding, and one's comfort zone as far as fast-wearing "grippiness" vs. long-wearing economy with maybe less peg-scraping aptitude. I nurse along an old Iron Belly Bullet, so clearly testicle-retracting performance isn't as high on my list as good hard wearing rubber that I'll have to change as infrequently as possible to get me through the annual state inspections. I was happy enough running those OEM Avon Speedmaster IIs (aka. "Skidmasters") right on down to the cords, but she's now wearing a pair of those retro-looking Dunlop "Good Seal" K70s, which have been a superb fit for my rather sedate following-Grandma-to-the-doctor driving style. I've long favored those reasonably hard-wearing but tractable Dunlops on my Norton twins, and suspect that I'd be likely to shod any future Interceptor I might own in the same, once those OEM Pirellis wore out. A good deal for them can always be found through the online vendor Bike Bandit.
Speaking of those OEM Pirellis, I am led to understand from some online review I can no longer now exactly recall, that the ones shipping on the new 650s are actually specially made to slightly lesser specs for Enfield than the "real original Pirellis", which, among other aspects, are said to have a tad more tread depth and hence wear a bit longer. So, of you were happy enough with the OEM ones, you might want to give the "real" ones a try.