My favorite? (Note: all I do is range target shooting)
9mm. Low cost, moderate recoil, use it in my 995 carbine.
Then .22LR. Lower cost, lower recoil, use it in my Ruger Mark III Target for Speed Steel competitions.
However, my son and daughter both REALLY enjoy shooting the 7.62x54R out of our Mosin...
(this may all change soon, too, seeing as I just bought a Hi-Point JHP (.45 ACP) and a Ruger GP-100 (.357/.38)...