I heard Tower Hill Trading was pretty good and that they have great rates. The owners are good honest people and from India.
CTC, DRW, TRANSMARKET are well regarded in Chicago edit: you're looking for equity, the above firms are options and futures firms too. They are not the typical equity prop that yoou may have been referring.
old thread about Tower Hill: http://www.elitetrader.com/vb/showthread.php?s=&threadid=109874&highlight=tower+hill
I find it hard to believe that there are better rates then this. $6.00/1000 is very good even with no pass through rebates. I would be very surprised to hear of any rates lower than this.