I like how Metal Sonic has a bit of battle damage. The cracked eye screen and mouth plate just looks cool. Inherently, my obvious Metal Sonic bias makes me want to say that's my favorite here, but I think Doctor Doom wins out. The way you shaded the metal just looks so shiny and pristine. It's really nicely done.
Beyond those two, I really dig that Mr. Freeze design. The way it incorporates the hard shadowed Batman Beyond styled face is just the icing on-top. Just realized what I typed there and I think I'll stop before I go on a full Ahnuld style ice pun blizzard-- DAMMIT.
Seriously, though, great selection of drawings here.