My favorite combat handgun ever is the HK USP Tactical .45 ACP. I shot it once and fell in love, the recoil is ridiculously light compared to the Springfield GI I fired. It's also very reliable and packs 12 rounds, more than enough. The large grip fits my hands perfectly, as I'm a fairly large guy. It was tons of fun shooting with the KAC suppressor as well.
My next choice would have to be a GLOCK 20, it's very powerful and the recoil is not too harsh for me at all. 10mm Auto is expensive, but it sure is versatile and it has a nice flat trajectory.
If forced to choose between a M9 and GLOCK 17, I'd choose the GLOCK because I've never even touched a Beretta pistol before. Honestly, though, I wouldn't trust my life to a 9mm if I was being forced to use non-expanding bullets.