Canon EF-S 17-55mm VS Sigma 17-50mm VS Tamron 17-50mm?

Portrait != thin DOF!!!!  Vast majority of portraits taken at studios are shot stopped down.  The 70-200 f2.8 is a great portrait lens, and it's f2.8.  The ability to control the DOF is quite useful at times, but it isn't the be all/end all of a portrait.  If you need to take a portrait of one person holding only  while they are reasonably still with a bad background then f1.2 and f1.4 can be quite nice to have, otherwise it's not super useful.  Low light and event photography benefit more from it than "portrait" photography.  I don't think the 85 L wide open would be anyone's choice for a group portrait, for instance.

I've taken quite nice portraits with my Sigma 17-50, Sigma 50 f1.4, the 85 L, the 70-200 L, even the 100-400 L, believe it or not.