Best clutch "kit" can be as subjective and opinionated as best engine oil, best truck manufacturer, etc.
Many on here support and appreciate Coop45 as Arnie seems to be a no BS kind of guy. I run a Coop45 primary sheave and if I found I needed another machined sheave, Coop45 is who I would turn to again. There are other primary sheave machinists out there who put together bit clutch kit packages and it seems some of that within the clutch kit is just not needed and drives the price up.
What many do is have the machined primary sheave, very small amount of shim or no shim and then go with a stronger secondary spring. I don't ride mud holes, only trails and that combo works very well for me on my 2014 Grizzly 700 with 26" tire size.