I'm going to have to go with the Nemesis Dreadknight. I know a lot of people dislike the 'baby carrier', but I already have two of them, and neither are based around Grey Knights; one is my Chaos Contemptor Dreadnought, and the other one is heavily converted to be a Riptide for my Adeptus Mechanicus counts-as. I also intend to use parts from more of them for other upgrades; I like rear-bending walker legs and the Dreadknight is easy to do that with for bulked up Dreadnoughts, etc.
Oh, and @MossyToes: I like your Heldrake. I actually have a Heldrake similar to that; I used a 3d-printed tail instead, but I have two Necrosphinx tails sitting here from before I fund the CAD design.