Without seeming too pedantic, if you use a firearm for SD, then you had better already have "the gun you must have" - in other words, if you haven't selected properly, then you have problems.
at the moment, the guns at the top of my "must buy soon" list include such products as:
RUGER SP101 .357
NAA GUARDIAN (have yet to decide between .32NAA or .380)
SOME NEW 1911 (either a SPRINGER or a COLT)
and a new GLOCK (either 29 or 33)
I would love to find any of these (in no particular order):
SIG P210
Colt Python pre 1970 with a 4 in barrel
Colt diamondback 22 with 2.5 in barrel
S&W pre war registered magnum 5 in
S&W 627-5 with good grips
Of course none of these are especially rare, I just want to find them for the right price. These guns are all valuable and I'm more of a bargain hunter.