What ever the engine is happiest at.
I've been through an iterative loop before, richening AFR's, closing throttle more to drop idle speed back down, tweaking ign timing, again closing throttle more. All edging towards getting maximum vacuum at idle - then settle somewhere around that particular AFR.
I have one car that idles perfectly at 12.5:1. Off idle or part throttle things are way leaner but that 12.5 gives me max vacuum and smoothest idle.
There is nothing to say it has to be anything like 14.7 Also what your trying to do in terms of emissions makes a difference as well.