Sure, I'll jump on that band wagon.
Vocals: Bruce Dickinson
He's got it all. He might be the best rock singer alive, and except for harsh vocals, he can do pretty much everything that I care for. He can hit all those high notes without resorting to shrieking, and his voice doesn't sound like a squeezed muppet either. He is an amazing front man and writes some of the best lyrics out there.
Guitars: Dave Murray
It seems like that whatever he touches turns to gold. A single guitar lick of his can make a song magical already. His riffs are unique and his solos are out of this world.
Guitars: Adrian Smith
More technical than Dave Murray, but not an ounce less feeling. What I like about him is also the fact that he is very open to different influences and although you can always recognise his handwriting, what he does is never the same.
Guitars: Janick Gers
I know that three guitarists in a band is untypical, but if all three can add something distinct and recognisable to this "all star" band's sound, why not? In his early days he was a rather sloppy guitar player, but he has really evolved into an incredible musician, both live and in the studio, and an amazing song writer. Any "all star band" should have a master in the songwriting department, and I don't think they could do better than Janick Gers.
Bass: Steve Harris
Probably the best bassist out there - although Geezer Butler is very close. You could be forgiven for mistaking his bass for a lead guitar. I think this is a particularly difficult instrument to develop a distinct and recognisable sound with, but Steve Harris does it. He is also an outstanding song writer, probably the best there is.
Drums: Nicko McBrain
It's quite amazing what he can do with his drums. Just listen to the heaviness of his playing, or the speed he reaches - without a double bass!
These six guys would probably constitute the best band ever. Just for fun, I'll add the runners up:
Vocals: Ronnie James Dio
Rest in peace. Since no rules were posted, I'll make up my own and say that deceased musicians can join too. I don't think I have to say very much about him- he was an amazing singer, a charismatic front man and an incredible song writer. Nobody sounded like him, although many tried.
Guitars: Tony Iommi
Like Dio, this guy is no ordinary musician. His playing and writing is borderlining divinity. Not only did he invent the heavy metal riff, but he also excels at it like none other. I also would believe that, at least in the metal world, his soloing is the most imitated, and yet nobody has ever matched him. Putting a second guitarist next to him is an insult, so I'll skip that.
Bass: Geezer Butler
One of the most recognisable bass players ever, and also one of the very best. I think he'd manage to add something distinguishable to any "all star band".
Drums: Vinnie Appice
It's a tough call between Appice and Bill Ward. Both are excellent drummers, but I think that Appice would fit in better with the rest of the lineup of this band.
...and because I still think I left out some extraordinary musicians who deserve mention, here's a third lineup.
Vocals: Freddie Mercury
He was probably the best rock singer who ever lived. Of the individuals mentioned so far, he's the first whom I never saw live, but judging from the ample live footage available, and from the many reviews out there, he was a one of a kind stage person, who managed to communicate even with the very last rows in a sold-out stadium.
Guitars: Brian May
I have always felt that Brian May is an underrated guitarist. He's never done extraordinarily long solos, but soloing isn't everything that guitar playing is about (although there are a good number of solos out there that prove his excellence on that level). His riffs and rhythms are instantly recognisable. He is also a great song writer and singer.
Bass: John Deacon
Okay, Deaky is a bit of a stretch. He often seems to be invisible, but if you do pay attention to him, his playing is precise as a Swiss clockwork. With all the released output featuring him, studio and live, there is not a single glitch - not one. Also, I think his song writing skills deserve more recognition.
Drums: Roger Taylor
Again, Roger Taylor isn't often recognised as a particularly remarkable drummer. I have seen him live however, and he does excel onstage. Also, he has been known to sing challenging lead numbers while drumming, without missing a single beat. I guess that belongs to the "not a lot of people know" category... and yet, is one of the most impressive skills imaginable.
So yeah, what I listed up there would probably be the three best bands ever.