IF your definition of best is 100 yard groups ( naturally you don't care the cost) out of the box with standard ammo, Rapidairweapons HM1000x gen 2 .30. American made ( not that I care, I just want MOA) barrel, assembled and tested by the company owner, each and everyone.
The Flex may well be good, I haven't personally seen one tho. Also I am not sure there are more than 10 production unit out there. It seems their plan is to build 10 per month.
And as it is a brand new rig time has yet to tell though Jim is a pretty good guy so as long as things go well it is very likely he will be in business a good while.
There was supposed to be a Slayer ( from American arms ?) but I don't believe it really exists in a production sense.
Non of the Korean or such would quailfie as "Best"
Maybe the FX Boss? If going FX I would certainly go for a model that has been out a while.
And there are one or two tiny manufactures.
If wanting , best, in .30 you might think of a used rig that has placed well in competition, there was one for sale here last week. You can often save a lot of money buying used from folks who shoot competition, "I" have NO idea why a rifle that placed 1st, 2nd, 3rd at a national event ( any cal.) sit's on the classifieds for more than a day but they certainly do.
John