I think order of publication. The last books were a sort of prequel and written accordingly.
Amazing series!
Read the first three in publication order. Not sure how to reas after that as I stopped reading on the forth book as I found it dull, although I may pick it back up one day.
I read in order of publication! Seemed to work well.
I read Foundation, then the two prequels, then everything else by publication date. I remember this working well, but this was around 20 years ago :)