It is more difficult to pinpoint metal bands that are universally respected amongst true metal fans compared with classic rock because there are more bands that are more widely known to contend with nowadays (thanks to advances in mass media). Although, I will say, yes, there does exist a great number of classic rock bands that aren't as well known and therefore not as respected (ie: Coven (not the metal band Coven, but the one with Jinx Dawson).
It is easier to break it down into the respective sub-genres of metal.
When it comes to thrash for instance, the first bands that come to mind for me are Slayer, Kreator, and Overkill. This doesn't mean that these are the best bands in the history of the thrash metal universe, but these are bands well-versed in thrash that seem to be the ones thought of by many when you think of thrash metal. So, can't say they are necessarily more respected so much as they are more widely known.
More "classic" metal bands that come to mind are Black Sabbath, Iron Maiden, Judas Priest, Dio. Again, for the most part these names are well-known and recognizeable amongst anyone who likes metal... but does that necessarily mean universally respected?
Death metal bands I think of Death, Morbid Angel, Deicide, Suffocation, Obituary.
All of these names are recognizeable to most anyone who likes metal in general. And if they aren't, then I would take it that person should try and open their musical horizons just a bit (but come on, are there really metal fans out there who have no clue about Death??!?... I couldn't believe that when someone posted that higher up in this thread).
I think people are going to lean towards naming the bands that are more widely known in regards to naming ones that are most universally respected, but I don't necessarily believe that means they are most universally respected. The interesting thing is... when you think of the classic rock bands that are "most universally respected" they (for the most part... not a general rule, take Aerosmith for example, and despite the lineup changes and even change in style, Pink Floyd is pretty much universally respected) had short-lived careers either thanks to an early death of a key band member or just chaos amongst its members. When it comes to metal... there are many bands out there that have been around for years, but few and far between have had the same lineup consistently or have even kept up with the same style of metal they play.
I need sleep... I'm sure I'll re-read this in the morning and just think "WTF was I talking about" haha.
Anyway, I don't think there are many universally respected bands, at least not nowadays with every other person stroking it to solely one genre and sticking their nose up at every little different technical difference between the subgenres. Hell, as long as the person at the mic isn't whining I'll take it so long as it sounds good and makes me want to kill (just kidding).