I think Ubuntu Pro is more common in service providers that sell to enterprises rather than in enterprises themselves. It enables them to say "yes, we comply with all of these box-ticking standards that you require your vendors to have!" without bringing in much of the rest of the enterprise baggage.

SuSE is used more heavily than any of them - as others have said, they're used more or less everywhere where SAP is to be found, and they're very strong in the HPC space too.