Higbean: I have been Hunting all manner of Varmints (and predators) for 60 years now (I am 69 soon to be 70!) and I have used them ALL, in my pursuits.
Including some Ackley Improved versions of factory cartridges along with some "wildcats".
I won't bother to list all the calibers of Varmint guns I have owned and used in the past - just suffice it to say I currently own 92 (ninety two) centerfire caliber Varmint Rifles and 39 (thirty nine) rimfire calibered Varmint Rifles
If I were starting out today and wanted an "all around" Varmint cartridge that will do 99% of all Varmint and predator Hunting and do it VERY WELL I would not hesitate an instant, to declare the 204 Ruger to be that cartridge!
Day time, night time, close range, rather long range, in extreme cold or in extreme heat the AMAZING versatility and wonderful attributes of the 204 Ruger make it my clear choice for "best Varmint cartridge of all time" (all around Varmint cartridge!)!
Just a few of the wonderful attributes of the 204 Rugers for your consideration as best Varmint cartridge of all time:
#1: Amazing accuracy!
#2: Amazing lack of recoil (this helps in all Varminting situations but especially helpful while spotlighting at night).
#3: Slow to heat a barrel.
#4: Ease of reloading (and economical to reload).
#5: Amazingly flat trajectory over distances where 99%+ of all Varmints (and predators) are shot.
#6: Excellent wind bucking ability over distances where 99%= of all Varmints (and predators) are shot.
#7: Excellent selection of 20 caliber high quality (accurate and lethal!) bullets to choose from.
Long live the amazing 204 Ruger!
Hold into the wind
VarmintGuy