I haven't looked into this for well over a year (which is naughty I know as I should do this for my product reviews, will get round to it lol) but when I did look into it Livefyre was the one that came out top. I don't know what it is like now or what is currently out there but thought this may help.
One of the issues I came across when looking for them was mixing the comments together so you could login to the site through wordpress and social media, many were an either or. This meant that you also with many didn't retain the comments on your site, they sat solely on the third party application ie facebook. This one offered both at the same time, integrated well and imported the comments into wordpress.
I know I am too late to the thread for you but thought it was still worth posting.