Which cartridge can call itself number 1?
There's a lot of misinformation in this thread. The fastest factory loaded cartridge is the .223 WSSM. Here's one reference to those indicating the
Swift for the inevitable naysayer, and if you google, you'll find even more with that conclusion.
I'd clearly prefer .22-250 AI (not factory) over either the .223 WSSM (barrel burner with bad brass), Swift (burner rep, and serious case abuser), or .22-250 (stretches brass too.) But for a factory cartridge, you'll do fine with .204 Ruger or .22-250.
If you want real speed, and you reload, .22-243 Middlestead. It'll use barrels like .223 WSSM and the Swift, but move things along much quicker. There are Wildcats, like the .30-06 case necked down, that may push things faster, but then you're deep in diminishing returns territory.
Again, to be practical, most would be better off choosing the .204 Ruger or .22-250 for factory ammo. If you just have to have speed and you reload, .22-250 AI or .22-243 Middlestead would be better choices. The .22-250 AI runs neck and neck with the Swift but reduces the case stretch (and some say wears barrels slower in the throat due to the 40 degree shoulder.)