That is one of those questions where you will get a lot of answers that are all pretty much opinion. IMO (since we are dealing in opinions), the best defensive ammunition for any handgun is:
2- Feed Reliable in your gun(s)
3- Accurate in your gun(s) and hits point of aim
I'm pretty much happy with Speer Gold Dot or Winchester SXT, Ranger T, PDX, etc. but that doesn't make the others bad choices. Any premium JHP should do the job in .40 S&W (and probably the regular JHP ammunition too). In .380 ACP my preference (if I had a gun so chambered) would be a solid slug of some kind to increase penetration. My choice would probably be Buffalo Bore LFN.