Freddie was easily one of the best showman in the history of music, but one of the best singers of our time? Not even close. For my money, Alice Cooper is the greatest showman of all time. Plain and simple, he inveted what the rock show is today. However, he should never be considered as a "great" singer. There is a difference.
As for putting Queen up there with Led Zepplin, obviously they're influence is probably just as far reaching but they're two totally different styles. IMO that makes them pretty hard to compare. Zepplin's influence is certainly more rooted in todays hard rock/metal which is probably why I would take them over Queen any day of the week. Queens influence tends to lend itself more to the pop-rock style which just doesn't do it for me.
However, I will say that Metallica's cover of "Stone Cold Crazy" is a great jam. I wonder what Freddie thought of that version......