I think I visit this drawing at least once a week (yes, I stalk some of your illustrations). It's by far one of my favorites, which is saying a lot considering you have so many great illustrations AND, in all honesty, I'm not a huge Tarkus fan like so many others. I think my love for this drawing has a lot less to do with the subjectivity of it and more to do with simple aesthetics. It has a brilliant use of what is usually referred to as a "triangle composition;" the perspective of the Iron Golem in combination with the posture of Tarkus creates a nice pyramid shape and the Iron Golem's right arm even angles-in nicely to reinforce the triangle composition as a whole. Beyond the composition, I can't help but love looking at it abstractly; the curvature and flow of those shapes that make-up the Iron Golem's shadows on Its armor is fun to look at alone.
Overall, it's just such a brilliant piece, I love it.