three of my top six favourite Pokemon are steel types in different gens, so it boils down to how good the Pokemon "supporting" them are.
and I think gen 3 just edges out the win.
I don't feel strongly about any of them, truth be told, but both Mawile and the Aron line are in that category of "I hope to use these Pokemon in the future". Hope that Mega Mawile is easy to get in Z-A ... Jirachi and Registeel are cool to look at which is enough for me.