I have two Praetorian Sentinels (GW) that I use. I have been looking for a Killa Kan at a reasonable price for another 28mm walker. I also have a converted Cadian sentinel for French forces, with a long spar torpedo. (I didn't make it, but I forget off the top of my head from whom I bought it.)
I am also looking at the Dystopian Legions Tuetonic Knight, and the Dystopian Wars Metzger and Shildtrager robots.
I have a 15mm from Black Hat, too.