Free: Contests & Raffles.
It has been covered pretty well above but terrain and expected shot distance play a huge role. I have more varmint calibers than anything else (creates a dilemma every time I go out to pick which one!) as I have .17 WSM, .204 Ruger, .222, .223, 22-250 and .220 Swift. The 22-250 and Swift are my favorites as they are good for any range I want to shoot but they are both heavy. I have poked coyotes at close to 300 yards with the .204, .222 and .223 but they very rarely drop dead right there.If you were going to have just one I would go with the 22-250 or Swift. .243 with lighter bullets is also good and is actually a more all-around rifle as you can hunt deer with it with heavier bullets.