From a singer/frontman perspective:
Steven Tyler is definitely the most consistent one, easily. In general, he's one of the best-aged vocalists in rock. Sure, you could say that Jagger still has his voice, but Stones song are mostly easy to sing, so he didn't have to blow his pipes every night for the last 40 years like Steven. Sure, Aerosmith shows aren't long, but Tyler is giving his all. Live he can still do justice on pretty much the entire band's catalogue, which at his age is saying something.
I like the Axl hit and miss description. He's hit and miss, times 10. In his prime, an absolute beast, as a frontman he's as intense as it gets. In 2010 I felt he was the best Axl he could possibly be vocally (I don't it's possible for him to sing any better than
), but the they had this ~10 month break where his voice for some reason returned to 2001/02 form, except with a little more rasp. Although he still can show some magic (
), so I guess for him it depends on whether he wants to use the raspy voice in the last couple of years he uses it rarely, I wish he did it like in 2010 and then I'm pretty happy. Will see how that goes. As a frontman, he'd still have it if he wasn't running backstage every solo to pump oxygen. Anyway, he's the only one of the three I haven't seen live, I'm still waiting for that.
With Jon I have at least to say. Completely lost his higher register. Cut down on his movement on stage. Yet for some reason I still think he's the best frontman of the 3. I don't know if it's the unpredictability in the sets (you don't have a feeling that anything can be played in Aerosmith or GNR shows), or his rare ad-libs, or an unexpeced high note in a ballad. Even at his age, I feel that he's able to give the best shows with less effort in terms of singing and moving, but he just has IT. Sure, it depends on his mood too much, but when Jon's in it, there's not a better show on earth. And I was very satisfied with his vocal shape in 2010, unlike some people here.
As songwriters, they're all very different obviously, but I feel that Steven is below the other 2. For me Music From Another Dimension has 2 types of songs: borring songs and sugar-coated ballads. Honestly, Jaded is the only really strong song IMO that they did after Get A Grip (Miss a Thing doesn't count as the band had nothing to do with the writing of this one). This album (Music...) and Chinese Democracy are not that different when you think abou it. Music... was their first original in 11 years and Democracy was first in 15. The former was probably better received because it had the same people in the band and it wasn't so different from a genre perspective, but from a songwriting standpoing, Democracy shits all over it. With Jovi recent efforts is different, cause Jon still puts out so much music, which at his age usually leads to decline in quality. But I love Lost Highway and all the other 00s albums would still have enough great tracks to make into one big superb album. So Jon wins in my book, but as Crashed pointed out, this is an obvious choice in this board, I just felt like elaborating on this one.