To answer the asked question, FRONTMAN:
1. David Lee Roth
2. Steven Tyler
3. Axl Rose
It's all relative, though; I think all three are upper eschelon.
To answer the unasked question, SINGER:
1. Steven Tyler
2. Axl Rose
3. David Lee Roth
Honestly, though, I think all three are really good singers. I've seen Steven Tyler a number of times (they used to be a "go see whenever they came through town" band when I was married to my first wife) and even in the show where he was wasted and fell off the drum riser and needed to be carried off stage, he still sounded really good in terms of delivering what was asked of him vocally. His voice has also deteriorated a lot less than most of his peers.
I've written already about seeing Rose twice; the first time was a visceral experience and one of the best "rock concerts" in the mythological sense of the phrase that I've ever seen. They were DANGEROUS, and there was this real feeling of "anything can and just might happen" during the show. The second time - on the Chinese Democracy tour, in a club - was a vocal tour de force. Over three hours, and he sang almost the entire time, at a high level.
I also think David Lee Roth is a way better singer than he gets credit for. Not in a Presley/Pavarotti way, but he has a fairly broad range, he has a multitude of different voices, and he brings a fair amount of variety to the table. I've seen him twice, and while both were spectacles more about the show than the technique, he still acquitted himself well, holding his own with some of the best musicians on the planet (Vai, Sheehan, Van Halen, Van Halen).