Choosing only one, what center fire caliber do you shoot the most?
My favorite would be shooting a 38 spl 158 gr SWC in revolver or rifle. They're also very economical to shoot yet versatile. I doubt I'll ever make a trip to the range without taking one with me.
I do enjoy various other larger calibers but shooting 38's out of one of the 357 revolvers is popular around here.