I think SPG/Marriott with the appropriate credit cards wins by a country mile! There's no other chain that currently even compares in terms of coverage or point value.

But, that could depend on one's usual destinations and targeted redemption properties. IHG and Hilton have great properties in certain locations, so one of those aligns well with your tavel patterns that could be the way to go. The downside to Hilton and IHG is that in many locations they only have lower mid range properties and limited service properties, whereas SPG/Marriott/R-C have better upscale and luxury properties in many locations, without doubt.

Or, just get mid tier status at multiple chains via credit cards and diversify.