I find Di'Anno to be just the right singer for Maiden's first two albums. Obviously, Dickinson was what they needed to bring it to the next level, beyond the confines of NWOBHM.
But there's such a charm to these two albums that really sets them apart from contemporaries of that scene, I think, and Di'Anno certainly has a big role in that. Prowler, Remember Tomorrow, Running Free, Phantom, Murders in the Rue Morgue, Killers, Purgatory (that's one of my sleeper favorites from the first two, actually)...he's just perfect in those...