Depends on the varmint on what is my favorite round.

Ground squirrels: Custom 10/22 with Shilen barrel .22lr and Savage BTVSS in .17hmr for anything past 60 yds going 1260fps and 2600fps (velocities pretty close to listed on ammo boxes)
Prairie Dogs: AR 24" White Oak Armament and Timney trigger .223/5.56 barrel in 1:12 twist Vmax or Nosler balistic tip 40 grain and working on Zombie 50 grain now that should rival the accuracy with most shots under 0.5 MOA going 3400fps
Rock Chucks: Remington 700 varmint barrel with Timney trigger in .243 and 58 grain vmax with one load nearly 3700fps and another almost 3800fps and very accurate