What defines "best"?
I'm very fond of .17s. If I were being strictly practical, I'd keep my current .17 HMR and own nothing else smaller than a .25-'06. The distances here are generally short and most of the shooting is at rodents slightly smaller than a prairie dog. The number of shots fired at things the .17 HMR won't handle well are few enough that using a .25-'06 for them doesn't create any issues....
...If I lived somewhere else with a different mix of varmint sizes and / or different shooting distances, the answer would surely be different.
I think my next varmint rifle project will being life as a .223 and get rebarreled to .17 Remington.
Tom
A good, sensible answer. The primary consideration is what you are using it for. That changes with the region.
I bought a 223, but don't use it. I have two 222s, and they do everything for this area. Coyotes, groundhogs (the tubby marmots - up to 6 lb.) and foxes. I've tried all kinds of bullets, but settled on 40 gr. Sierra BKs. Load 'em. Shoot 'em. This bullet works for everything. I have never had a 300 yd shot where I'm living now, and this bullet with H4198 is a 250 yd everything gun. Like you said, Tom, it does everything for where I live. And that's the key.
There was a time when I tried other cartridges, but got rid of them all. My 222 Rem load develops 48k pressure, according to my computer, which is supported by the fact that case stretching is a non-issue. 3/4"@100. It's golden. But may not suit the requirements of others.
Edited to add: around here, 223 brass was made to be rechambered to 6x45mm. It seems better suited for the purpose.
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