Nowadays I use mostly a 9 mm (Kahr PM9) and two .45 ACPs, a SIG Revolution STX( its full size 1911 type) and a SIG Revolution Compact(its a Commander size slide on an Officers Model frame 1911 type.) Those two are my current favorite SD pistols, but I have various revolvers, Glocks, other 1911s, BHP, SAA, etc, etc that I would feel comfortable using for defense.
Thinking about it though I would say my #1 favorite SD pistol right now is my .45 ACP SIG Revolution Compact, I always thought the Commander size 1911 style pistol offered the best in portability, accuracy and power, but owning and using one with the Commander slide on an Officers Model frame has caused me to rethink that.
My 2nd favorite has to be my Kahr PM9, it is very compact, powerful and accurate.
As to caliber, anything from .38 Sp to .45 ACP is suitable for defense, if it is used in a reliable pistol, that you shoot accurately.
The calibers and cartridges smaller than .38 Special, the .380 ACP, .32 ACP, etc will work for defense, but I personally prefer something bigger.