Tough call.
One 9mm for
what?
Range/leisure/target shooting? 5906PC
Duty/Service sidearm?
Metal? 5903
Plastic? M&P 9 (Runners up plastic? G17, followed by USP9)
Lawfully concealed carry? Ah, this is where it starts to level out for me.
I chose
other.
I tend to want smaller/lighter 9's for my needs, which is now mostly only retirement CCW. I spent enough years having to carry larger & higher capacity 9's on-duty. Not a target/plinker, anymore, either.
While I own a couple G26's, which are fine little guns for basic plastic pistols, I have a couple other preferences for "long haul" 9mm's. I have a late production (end of 90's) 3913, a CS9 and a SW999c (think P99 9c AS).
The 3rd gen S&W's are reliable and more than accurate enough for my needs (I can use them to shoot wooden clothespins off the target backboards at 7-10 yds with any of the various duty loads we've issued.)
The 999c has a better trigger (granted, that's after having fired close to 15K rounds fired through it), and after an initial tolerance issue with a Walther trigger bar guide post (occasional light-strikes in DA mode, and it's an pretty uncommon tolerance issue to occur, FWIW), the gun has been completely reliable. It's even a bit more accurate when pushed, too (which I found interesting as I realized it over the years).
The 3913 & 999c are the 9's I choose when I'm going to be pushing myself (to check basics) for aimed precision fire out at 50-90+ yards. The G26's are also accurate, but they don't have quite the same level of precise triggers as the TDA-type pistols.
As an armorer for the Glock, S&W and P99/SW99 pistols, I find them all easy enough to maintain. Glock parts are somewhat easier, as the 3rd gen guns have been relegated to LE-only status and parts are getting a bit less available than previously. Walther parts? I guess we'll see how they do not that they've set up their own import business. Waiting for parts to arrive from Germany (to Walther America/S&W) used to be a bit unpredictable.
The Sig 9's are pretty decent, and I almost bought a P239 9mm when I went through the Sig pistol armorer class, but they thought very dearly of the 239 when it came to individual officer pricing for their chunky little single stack.
Good guns, though. Spring-dependent (number of springs & longevity of some of them), and they clearly tell their armorers that they're "wet" guns when it comes to sufficient lubrication. If I didn't already have some nice compact/subcompact 9's, and the 239's were more reasonably priced, I'd own one.
So ...
other, it was.