Steve Harris IS Iron Maiden. The thing I loved about Maiden was that all of their old stuff was quality work, and there was A LOT of it. How many albums did they put out before Dickinson left? Debut, Killers, Beast, Piece of Mind, Powerslave, Somewhere in Time, Seventh Son, No Prayer, Fear of the Dark, all quality albums, all a little bit different yet they maintained the same Maiden-esque feel to them. To put out that much quality work is pretty rare. My old roommate was a HUGE Maiden fan, and also a very, very good bassist (Rush and Dream Theater he was especially good at playing along with), and he found Harris' bass playing to be pretty challenging. Unfortunately, Maiden gets lumped into the 80's crap metal genre, which they definitely don't deserve to be included in.
*edit* As for greatest metal band of all time? I dunno, I'd say that honor goes to Metallica. OLD Metallica.