Do you want a commercial caliber or a wildcat?
Wildcats you can do sooooooo many different calibers your head will ache for weeks.
Commercial calibers are many and the prices are all over the board, it all depends on how deep your pockets are for the build.
Personally I feel the 243WSSM is one of the finest commercial calibers available for the AR platform but are only offered thru very limited sources. Oly Arms, Dedicated Technologies and AR15 Performance, also Dan Carey. all build exceptional AR's.
The 300 blackout is a newer rendition of the old 300 Whisper, it's main focus is suppressed loads and not conducive to long range shooting.
There is the 6WOA which is available from White Oak Armament that does a great job of getting out there to 400 yards or more accurately.
The 6.5 Grendel and 6.8 SPC are both very popular and will do well out to 300 yards but then the 6.5 Grendel really shines.
The list goes on and on, some have little issues you need to learn to deal with, others work very well but only really excel with longer barrels and so on like the 204Ruger, just use at least a 22 inch barrel and you'll do well.
Oly Arms now offers a new version AR that shoots the 22-250 round, not much for reviews out there yet but the 220-250 is a proud varminter round and has a great following.