In my opinion, 22s are the most addictive, especially higher end guns that are accurate (S&W K22, M17, Ruger Single Six, Standards, MK IIs, IIIs, 10/22, Browning Buckmarks, CZ 452, etc) at today's prices, you can shoot over 500 rounds for under $20! Everyone, regardless of age, size or strength can handle the recoil and learn to hit what they are shooting at, quickly. What other caliber guns can you do that with?
HOWEVER- my favorite to shoot are 1911 style pistols and either 1903 or 1903A3 rifles