It all comes down to personal preference too. I prefer .30 caliber guns over anything else because .308" diameter guns are just about perfect. I read in a Field & Stream artical awhile back called "The 500yd big game rifle is a reality" and they said a bullet bigger than .30 caliber wouldnt have the ballistic coeficient or aerodynamics and a bullet smaller than .30 caliber wouldnt have the power out to longer ranges.

Look at what diameter the most popular calibers are nowdays.
.30-30 Winchester
.308 Winchester
.30-06 Springfield
.300 Win Mag
.300 WSM
And so on....