Best current back to back streak:

[Batman]

Dillian White is racking up a decent record considering the poor division

Chisora
Helenius
Brown
Parker

Obviously not as strong as others on the list but he is definitely trying to work his way through those outside of wilder and AJ and as I said the division isn't as strong as others, you can only beat what is in front of you.

[TitoFan]

Well, if we're going back in time..... how about Bailey, Corley, Abdullaev, Torres. TBH, you could go through most of Cotto's career and just pick out the very few "soft spots" every now and then.