rackback is something like give u cash back when u pay some rake on either cash or tournaments buyin...
but most of the sites now only have points to cash system, this somewhat equals to rakeback....
like if you gain 500 points you may change to 5 bucks something like that, but exchange rate may be different, if you are in different level.
i honestly don't know what is the best sites for rakeback...
but u could compare with several sites.
you may do some investigation work and easily find it on site's webpage..
good luck...