My choice is Barbara Reynolds' translation
I have read and/or sampled William Stewart Rose's, Barbara Reynolds', Guido Waldman's and David R. Slavitt's. My favorite is Barbara Reynolds as the translator.
I first tried reading Orlando furioso by going the cheap and easy way of using a free online copy that was translated by Rose. I found it stiff, hard to understand, and confusing. I tried several cantos and found the process to be painful.
Then, I bought volume 1 by Reynolds and it was as if I had been in a darkened room and someone turned the light on. I liked reading it as a poem with rhymes. It wasn't sing-songy annoying that some poetry can be.
Later, I discovered Waldman's. it is adequate, but I find it has some physical drawbacks in that it has a tiny font and the paragraphs are harder to read than Reynolds' version with lots of easy to read white space.
With an epic story, like this making it easy on your eyes is something to consider besides just the text.
Slavitt's version has flaws as far as I am concerned. The biggest flaw is that his editor was concerned mostly by page length over content. He turned in a complete translation of the entire poem and they abridged it. There are many passages that flatter the patrons of Ariosto and those could have been cut without losing any important text. However the editor(s) went beyond that and eviscerated many subplots and merely summarized entire passages or even entire cantos. Some of my favorite parts of the poem were deleted.
I realized that early on as I was evaluating the book and gave it away to a friend, because I no longer wanted it in my house.
The ebook version appears to be an electronic reproduction of the abridged print edition, so I wouldn't recommend that either.