Which one, in your opinion, is already "perfect" in your eyes and couldn't have been designed any better, and why?
Depends on what you mean by common. To me it means being able to go into any store that sells ammo and be able to get a box of shells for that particular cartridge. First ones that come to mind are .223, .243, .270, 7mm Rem Mag, 30-30, .308, 30-06, .300 Win Mag, and maybe .338 Win Mag. I'd be very surprised if you couldn't walk into any store that sells ammo and pick up a box of those.
Of those, none are of the perfect design. We have learned some things over the years, but new is slow to catch on and I think it will be a while before things like the 6.5 Creedmoor are as common as say a .270. Less body taper, and sharper shoulder are nice. Minimal to no case stretching during sizing. One thing rifle manufacturers could do, is update their twist rates to stabilize modern VLD type bullets. No down side to doing so, and when ammo manufacturers follow suit, it would be a whole new world in capability. Been said many times here that bullets matter more than headstamps, and that is very true. The 6.5 Creedmoor, both rifles and factory ammo are way ahead of the game. Maybe in a few years the non-looneys will catch on and popularity will approach the old standbys.
But, if you want some serious ballistic capability and ammo availability anywhere right now, it might be pretty hard to beat a properly twisted .243, .270, or 7 Rem Mag. You would likely have to reload to take advantage of the really slippery bullets, but you could walk into a store and get ammo anywhere if need be.
John
