I can give you my opinions from a hobbyist's perspective.  Note that all of this is IMHO, YMMV, void where prohibited by law, yada yada yada.

Besides this site, of course, I stick mainly with Eros.  There seems to be a higher trust factor, and better class of provider, at Eros, though that's not 100% guaranteed (as I've learned through bitter experience, though it's mainly my fault for not doing my homework).

I avoid Backpage like the plague.  Every time my curiosity makes me check it out, it always comes across the same way: a hangout for sleazy women trying to get as much of your cash for as little effort as possible.  The place just screams "criminal element" to me.  I do occasionally see a provider there that I've also spotted here and on Eros, but that's about as rare as bipartisanship in Washington DC.

As for other sites, perhaps others can chime in with their opinions, as I'm still pretty much the n00b at this hobby.

This board is for bringing up legal issues, which is quite different from asking about the best places to advertise.