Perry Como. I think he's vastly underrated, simply because his career was sorta so steady, and he had a long happy healthy life and died in an old age. But, just listen to Como's "Fly Me to the Moon" and the Sinatra version and decide for yourself. Como and Sinatra. It could be a matter of my taste, though.
But, for a more modern dead artist. Melanie Thornton. Very popular singer in eurodance, sang for La Bouche in the 90s, among other eurodance artists. Anyway, at around the same time as the R&B artist Aliyah, she died in a plane crash like her. I personally like Melanie much better than Aliyah, and think she's one of the most talented vocalists ever to grace the genre of eurodance.
And lastly, as corny and obvious as this sounds, Michael Jackson. Musically, he really did sorta revolutionize pop music, even if the influence isn't quite as apparent in US, I think worldwide he had a LOT of influence. His first few albums were just flawless wonderful albums, and pretty much all his music pre "Bad" will be played a hundred or more years from now. The bandwagon that occurred after his death was completely nuts. Before he died, you almost had to be a closet Michael Jackson fan, if you were an MJ fan, you were immediately accused of being in cohorts with a child molester. You NEVER heard MJ on most radio stations after his whole 2000s case. Like overnight, my sisters's opinion on my limited amount of MJ music went from "you're weird, and it's old and lame" to overnight developing a massive appreciation of it magically or something. But, despite the bandwagon of people that now claim to have always been Michael Jackson's biggest fans, really, his music was spectacular, and nobody could really top him, I mean he certainly "faded away" as the years went on, but the first 3 albums can't really be topped. Now, too, it's ideal, as the craze has died down, so now not everyone and their dog is claiming to be MJ's greatest fans ever, but you can listen to his stuff without being accused of supporting a child molester. It's quite odd, in a sense, his death "freed" him, as now it's OK socially to listen to his music again. But yeah, MJ, obvious, but awesome.