Tactical nuke would beat the cement truck, hands down, for most knock-down power.
In the realm of firearms, it is all really based on what you can handle.
Top of the heap would be rifles chambered in heavy machinegun rounds, ala the barrett in .50 BMG
Next would be the superultramags using african game sized, things like 460 weatherby, 10.57 meteor, 500 A-Square, etc. next would be these same cases topped with 33-38 caliber bullets, Then I suppose the older, less high velocity african big game loads like 416 rigby and the like.
Of course, the number of people who can handle such a heavy recoiling firearm are pretty few and far between. Step down a notch to the 300 ultramags and the like, you still have one hell of a recoiling load, even the venerable 338 gives one solid kick. Now, the 338 is going to be excellent moose and grizzly medicine, and will sure as hell take any fight out of a 2 legged opponent if you place the shot on his torso, no matter how hopped up on dope or religion he is.
Then finally you get into your average true rifle loadings, things like 30-06, 8mm mauser, 303, etc, etc, and you can basically do this all the way down to 22 short and cb caps.
Handguns are pretty much the same thing, as far as standard production models, 500 S&W tops the chart, with such things as teh 50AE, 480 ruger, 454 casull following, but still well outperforming the 44 mag, Then I suppose-41 mag/10mm, -357/45acp, -40 S&W -9mm --38 specuial -380acp-32 H&R mag-32acp-25/22LR with lots of unmentioned ones in there somewhere too.
It all depends on who much gun you can handle. A 500 S&W too big to carry won't do you any good, and a 44 mag that recoils so much you cannot hit isn't going to be worth a damned, whereas a .22LR that yuo can hit with is going to be worht at least something.