The largest caliber handgun I can comfortably conceal for 8 hours that I can shoot most accurately, quickly, and that I can trust my life to; for me that's:
Glock 17, 9mm 124gr. Rem GS
Glock 26, same
Kel Tec .380 in Milt Sparks pocket holster; though it's not my favorite handgun to shoot it's surprisingly good for its diminutive size and weight, in sunny Arizona I find that it's the one handgun that I can always carry concealed whatever the weather or dress code.
What works for you is always good advice, but to those of you who carry in an ankle holster, make sure you practice a lot drawing from that position. I found it unworkable for me, and switched to pocket carry (with above mentioned Kel Tec) for backup or primary use. Also, if you choose to take a tactical pistol course, which you should, use the exact set-up (gun/holster/covering garment) during the course that you use in real life.