If you could only have one handgun for everything that you use a handgun for, which one would it be? (No sci-fi stuff lol)
Well, that is a lot to unload. As it stands there are several factors you have to think about.
SIZE:
You have to consider that, as you're looking for a good general purpose handgun, you're going to have to look at size and concealability (for IDPA, 3-gun and the like) as well as not being so small that it'll be a problem for you to handle.
MASS:
Light guns have more recoil and are typically less fun for normal people to shoot for any great duration. On the other hand, you still want to be able to hold it up for said extended duration range visits.
CALIBRE:
This is currently a double edged sword, as there isn't a whole lot of the "common" calibres available. But I'd still recommend looking to get a handgun chambered in a locally common calibre, such as 9 x 19 NATO or .40 S&W as per some police agencies.
UBIQUITY:
How common is the handgun? How available are spare parts for it? What about accessories?
So based on this set of criteria, where I'm located, I'd recommend looking at Glock 19, Glock 48, Glock 17, Glock 23 and Glock 22 in order of preference.
NOTE:
I'm not a Glock guy. I'm a "what's the criteria to meet" guy. If SIG's 320/365 series had better saturation in the Canadian market, I'd steer you to consider them as well.
Hope this helps.
-S.