I'm a big fan of the old metal models even though my army is pretty much completely plastic cadians. I'm not even sure what regiment I would like the most (even the metal catachans weren't "as" bad as the current ones). I am one of those players who loves an old skool guard army with several regiments mixed together as in ye olden days. But... if I really had to choose I guess it would be Valhallans, simply because every time some punk ork player start using the word waaagh you can outshout him "For ze motherlaaaaand"

. After that, I'd go for Tallarns or Steel Legion.