I was investigating this same question a few weeks ago. Although the Fahnestock and MacAfee translation (Signet Classic) seems to be the most commonly recommended, it isn't available as an ebook. However, I found the Hapgood translation quite readable. Although there are many free ebook versions of Hapgood's translation out there, I ended up buying the one from
Carefully Crafted Classics. I downloaded the sample from Amazon, and I was pleased by the fact that it restored the italics that are missing in the versions from PG and other free sources and that it expanded the first-letter-dash that Hugo used to the actual proper nouns: "Bishop of Digne" instead of "Bishop of D——" for example. I find it less distracting, a personal preference that made it worth the $2.99 for me.