The Glock 23 in .40 S&W was my very first handgun. The snappy .40 recoil out of that lightweight polymer pistol made learning to shoot handguns interesting. It really made me focus on getting down the basics (and the 200 round a week habit meant I was getting lots of practice.)
Later on, I found the .45 ACP to be rather gentle, in comparison, and the 9mm to have no recoil to speak of. These days, though, I shoot heavy .45 Colt, and find the recoil of the .44 Magnum to be refreshing. However, I place all blame for my gun collecting on that .40 caliber Glock.
But, I still have the Glock 23. I've got a CCW rig, lots of spare mags, and night sights for it. I've also gone through a couple of recoil springs.