I think out of all the band members, Edge would make the best solo album.
I think Bono COULD make a good one, but whether he WOULD or not is the question. When you're making a solo album, YOU are it. You don't have a band to back you up and if you're not doing anything, nothing is getting done. If you're in a band, and you have save-Africa end-Poverty duties to attend to, the band can work on the record to a certain degree in your absence. If it's your solo album, nothing is getting done when you're doing other business. Bono would REALLY have to commit to the project, spend more time in the studio then he's probably spent in the studio since the Pop sessions(imo), and put a concious effort into making the music sound unique from U2 to a certain extent. This is because, no matter what, a Bono solo album would always be compared to U2's stuff, and making a record that sounds just like a U2 record wouldn't help that. And honestly - after making music with U2 for so long without ever even hinting at a solo album - to make one at this point without everyone thinking he just wants more publicity and more ego-stroking would take a creative effort of a magnitude that I'm not sure Bono has in him in 2005. I'm not saying he doesn't, I'm just saying I'm not sure.
I think the really unique thing about U2 is, where in a band like the Beatles, Lennon, McCartney, and Harrison(heck even Ringo isn't horrible) could all hold their own and make solo music that at times was at least equal to Beatles music, the members of U2 really need each other to make the kind of music they make. I don't think anyone here can deny thinking that Bono + Studio guitarist just wouldn't be the same without Edge, or that Edge singing while playing a guitar - while I love him singing - just wouldn't be the same without Bono's ego-ridden listner-captivating singing, or that Adam and Larry, while being great at what they do, just aren't versatile enough to carry a whole load themselves. U2 need each other to make it work, I think.