Brian Williams said:
9mmx32R in a S&W 13 or 65.
+1...Make mine my 3" S&W 65LS or a 3" M13 if I can find one.
If you don't want that to count because you mean 9x19, I won't be happy.
If I had to do just one gun, it would have to be between my .40S&W SIG 229, my S&W 1911SC or my S&W 65LS. All have a lot more power available than any 9mm, are relatively concealable (they are not quite full service sized guns), with the exception of the revolver they are lightweight for their size (229 is aluminium, the 1911 is scandium), and they are large and heavy enough to be comfortable in extended range sessions.
If I had to pick a 9mm I currently own, again I wouldn't be very happy. Currently I own a CZ 75B and a SIG 226 in 9mm. Both are a bit big for an only gun. Even though I don't live in a CCW state, having a gun that is concealable is important to me (I am sometimes in PA and VA where it is allowed, you never know if life may see a move to a CCW state, and I like the option even around here in case crime ever got that bad that I'd ignore the law), yet if it is an only gun you want it to be big enough for a full grip (comfort) and heavy enough for extended range trips. So, midsize guns seem to be best to me. That said, if it had to be what I already own, it would be the 226 since it is a little lighter (aluminium) and shorter, thus a little more concealable.
If I could buy something new, my top contenders, more or less in order (though the SIGs and Kahrs are pretty much tied):
-9mm Commander sized 1911 from any of the bigger name makers (since I'm replacing everything else with this, maybe a custom), preferably in scandium or aluminium
-SIG 229 in 9mm
-SIG 239 in 9mm
-Kahr K9
-Kahr T9
-S&W M&P
-Browning/FN Hipower (the only full-size service pistol I'd consider- it is narrow enough to be pretty concealable)