Orantes has a case for no.1 in 1975, had a good summer particularly, but I think Arthur Ashe just has a better case winning WCT Dallas and Wimbledon. Orantes and Arthur both had their best year in 1975. Ashe won on more varied surfaces than Orantes overall.
It was a year no one person dominated.
Jimbo got to three Slam finals and lost the lot. He was the most consistent player overall, but didn't win a major, so Ashe is no.1 for me.