Well, it changes from time to time. Right now, and for the last few years it's been the 6mm BR.
I picked up a 6MM BR Remington 14 twist at a gun show a few years ago. The case on the Remington is .040 shorter than current 6mm BR Norma but the rest of the cartridge is the same. It will push the little 65 grain VMAX at around 3200 FPS and just smokes prairie dogs. Presently, I have a Nikon Buckmaster 6-18 scope on it and it works well out to around 600 yards. The downfall of that outfit is the slow twist, requiring light bullets and the 1/8 MOA adjustment of the scope.
You have 6 MOA of adjustment before you pass zero so to get out to long range, you have to pass zero a couple of times. Sooner or later, you're going to get confused and not get back to your real zero then get lost trying to hit anything. I just use it for 400 yards or closer.
Two years ago I had a 8 twist, no turn neck, 6mm BR Norma built. I topped it with a 6.5-20 Vari X 3. Shooting the 105 AMAX bullet, I can get out to 1000 yards with a little help from lady luck.
Another new favorite of mine is the .243 Ackley Improved. Not because I can squeeze more velocity from it but because the cases don't lengthen as much as the standard .243 so you don't have to trim as often. An 8 twist .243 AI with 105 AMAX bullets will get you to 1000 yards with range left over.
My varmint shooting started in the San Gabriel Valley of California before there were many freeways and a guy could walk around the foothills of Azusa and Glendora with a .22 rifle shooting jack rabbits and ground squirrels. Once I got a car I headed out to Hemit for ground squirrels and the Mohave for jackrabbits. I picked up a few Mauser rifles along the way converted to Varminters in .22-250, .243 and .257 Roberts for those shoots.
Then when the new Remington 700 V came out around 1967, I fell in love with the .222 Remington. About that same time, or a few years earlier, Redfield introduced the 3-9 Variable scope. That was almost perfect for my style of shooting. 400 yard shots were almost easy on those big jacks using the Hornady 50 SX bullets. I still have a rig that is almost like that old gun except it's in .223 using 50 VMAX and topped with a Leupold 3-9 scope. Just last week I made a number of 400 + shots with it on prairie dogs.
Guess that'll teach ya to ask an old duffer questions.