I regard Newton to have been a much greater physicist than Einstein was.
I regard Newton as a much greater physicist than Einstein was.
I regard Newton to have been a much greater physicist than Einstein was.
I regard Newton as a much greater physicist than Einstein was.
The verb "regard" idiomatically takes the preposition "as", never an infinitive. The idiom "with regard to x" is not appropriate in this case. This idiom is not used for considerations of people as in the example sentence. Use "with regard to" when considering or relating something.