^^^Very True Krosero. Borg's serve often gets overlooked. It was very good indeed, especially the first serve. He had such a beautiful motion, in my opinion, which was so efficient.
As far as Tanner, I would say that his serve is definite top 10 material. It's pretty much the only reason he was as successful as he was. The rest of his game was just not that spectacular. It was a stand alone, spectacular shot. Datacipher and PC1 are correct above, in my opinion. His delivery was key. The EXTREMELY quick ball toss and lightning quick motion, which made his serve impossible to read during the delivery. Then, he would deliver absolute bombs with that PDP! Of course he also had a lefty's advantage as well.
See last the 3 games of Tanner-Borg in the '79 Wimbledon Final:
Tanner's Serve Analyzed By Vic Braden:
(mentions Roscoe's 130-140 mph serve)
Borg-Tanner at Wimbledon in the 1976 SF: