Everyone will have their own favourite here and there really is no such thing as a bad calibre if used within the capabilities of the both rifle and shooter all will get the job done, my absolute favourite is the .17 Remington why ?, it has banjo string trajectory to 300yds and little or no recoil also it has really low ricochet potential I say low but not to be taken as non existence it needs the same respect as all calibres, I have shot many foxes over the years and believe me it is deadly.
Bad points are you do not have much choice of rifles or ammo out there you will need to reload to get the best from it, and build re-barrel an existing rifle to get up and running, some say its a barrel burner I am not so sure my current rifle a Remington 700 LVSF only just recently got a new tube after the old one had shot some 4000+ rounds and even then it still shot reasonably good groups.
Steve Kershaw replaced the old barrel with a LW50 light varmint barrel and boy does it shoot I have just coming up around the 100 rnd count and am sure it will last me for very many years thank you Steve you done a brilliant job sir.
My favourite bullet load combination are the Hornady 25grn v-max bullets superbly accurate and devastating on target, I have also had great success with Hornady/Berger 25grn HP bullets and the heavier Berger 30grn, but my bullet of choice are the 25grn v-max pushed along with Hodgdon Varget or Alliant RL15, I tend to keep my loads on the mild side around starting loads or just a little more I never go to maximum never exceed loads, they seem more accurate barrels last longer and it gives me a greater margin of safety.
Damian