My go to varmint killer is the .22-250. It is capable of reaching out and taking varmints at farther distances than many people give credit. I love slinging 55gr pills down range.
On that note, I just picked up a sporter rifle in .243 and love the ballistics. I may try and pick one up in .243 if I can find another good deal on a M700 varmint.
I am comfortable shooting out past 500yds with the .22-250. It still does what it needs to, even out past that.