Best....now that's pretty hard to nail down. Lots to consider however, if we're talking about varmints and cheap, accurate off the shelf ammo the list diminishes considerably. The 223 is about the cheapest and most plentiful ammo on the shelves for varmint shooting.
The best shooting 50 grain varmint ammo I've found is American Eagle 50 gr Tipped Varmint. These seem to be made with some unusually rare fairy dust. You won't find much info on the net for this ammo, and it's not on Federals website. I had to email Federal to get a spec sheet, and I am not exaggerating when I say that I have several targets shot at 100 yds with 3 and 5 shot groups measuring in the zeros out of an inexpensive totally stock (except for the stock) Savage model 12 FV.
Next is Australian Outback 69 SMK. Shoots very well and is quite deadly, despite Sierra's warning about not using match ammo for hunting.
Hornaday makes 75 grain loads in various velocities. If your barrel is an actual 9 twist or faster, or you're shooting above 3000 ft altitude, its golden.
Eventually as you start hand loading, the 223 offers plenty of options. It's not powder sensitive, eats most bullets with aplomb (great stability), has great barrel life and cheap components. It's a no-brainer for the first time hand loader.
The 53 grain V-Max at about 3300 fps in a 26 inch barrel will make 4 inch clay targets disappear at 500 yds just as fast as you can aim and shoot.