I settled on DK, FD and MGM after trying a few others as well(didn’t care for the others). Out of those three, DK has the most offers though a majority of them are tied to SGP’s. FD has less offers and their Boosts are also mostly tied to some type of parlay. MGM has fewer offers but the ones they do offer are straight money offers, able to use that money on almost any wager.
I doubt you’re going to find any book that is largely different than the others but the best thing to do is dl the ones that look interesting and experiment with them for a while, like I did. You’ll find the ones you like more than the others.