This discussion came up many times over the years on the SP forum and every time I found myself coming up with the same answer:
Most useful and essential are 223 and 44 Mag in Super 14 and 30-30 AI in a carbine.
Next tier would be a 6.5 or 257 Bullberry or JDJ chambering in a carbine, 357 Mag in a 10" and a 223 based wildcat in 14".
The 6.8 SPC has eased into the collection and has become a favorite but I wouldn't consider it a first choice over some of the others.
Since my hands won't handle much pistol recoil these days, I've gravitated to more carbine set-ups. That means that 223 is about the only handgun I shoot these days but 44 mag is still in the mix as a great carbine chambering. I've never seen rimfire rounds as particularly essential in Contender given the abundance of relatively cheap and accurate bolt and semi-auto rifles.