What's the 'best' caliber might depend an awful lot on desired range, target, action, economy and reloading equipment available. But, it could very well be that the most difficult thing to find, own, transport and process in the not-so-distant future is lead. Speculation aside...
From about the time I first became aware of the falling block action, I thought they were cool. They don't address any problem I have--I just think they're cool. From the time I became aware of the Ruger No 1, I've wanted one. I've also always been real interested in very big bore rifles as being 'fun', but extremely expensive to operate. I'd have a big Nitro Express gun if I could afford one, again, not for any imaginable purpose.
So, recently all these things came together in the form of a Ruger No 1 in .45-70, all for the purpose of slinging large-but-not-too-large slugs of lead downrange in a slow, relaxed manner, for no real purpose at all other than fun.
The economy vs massiveness tradeoff seemed answered by this round, and while folks have said it's not that accurate a cartridge, just as many seem to have said it's a very accurate cartridge. Hitting a 2L soda bottle at 100 yds is far more accurate than I really need, 'killing' the soda bottle isn't a concern, and quick follow-up shots likewise not a worry.
I like "45", and I like brass that's not two bucks a case...so I'm quite happy to work around any 'downsides' I may find, including shooting bigger targets if I have to.
I will find a suitably-accurate load that makes use of a cast bullet that I find appealing and that is a bit cheaper to shoot, do gas checks because I can, and sling those puppies downrange through the entire spectrum of velocities I deem 'fun'. That's my simple definition of 'best'.
One option I'm hopeful for is one at the light end of the range from Accurate that I think may be perfect, a 325 gr gem with no crimp groove:
