I thought Phil's first two albums (Face Value, Hello, I Must Be Going) were both excellent. Yeah, there was some poppy stuff, but also some pretty cool stuff that I don't know if you'd call it progressive, but it obviously wasn't aimed at commercial radio either. I know a lot of people cream their shorts over "In the Air Tonight", and I like that song, too, but it certainly isn't the epitome of his solo output or anything. And if you think about it really, it's nothing like a standard radio hit; it just happened to be in the right place at the right time. My favorite Phil Collins solo album is Hot Night in Paris by The Phil Collins Big Band. It is, however, an album of big band music, so not for everyone.
Mike + The Mechanics were always meant to be something like a Top 40 band. They get slightly adventurous sometimes, but I've never heard anything to sway my opinion. Good stuff, just not any more interesting than most commercial-grade stuff.
I would've thought Tony's solo output would be hands-down the best, but he really needs a good collaborator to bring out his best, and he's never really found one outside of Genesis. His first solo album A Curious Feeling is probably his best, but has dated keyboard sounds that bother even me (and I tend to be very forgiving of dated keyboard sounds). I wasn't that impressed by Bankstatement, an eponymous album from a band that's really just a Banks solo project. Same with Strictly, Inc., although I like that one a bit better. That's his collaboration with Jack Hues, the singer from Wang Chung. My favorite Tony Banks solo output is his neo-classical stuff, although I know that's not for everyone. Six and Seven are both great. Fully orchestrated stuff with all the Banks cool chord changes and stuff. A lot of it sounds like the instrumental parts of Genesis songs, which isn't surprising since Tony wrote nearly all of them. But I think it sounds amazing.
Peter Gabriel's solo stuff is very interesting, and I've heard a lot of people talk about how proggy it is, but really, I don't hear it. I have his first five, and they're good, but I don't find myself going to them very often.
Same with Steve Hackett. I love his guitar work, but that's about it. Most of the songs bore me, and I'm rarely impressed by the singers on his albums. If he did a solo album of just guitar/instrumental stuff, I'd probably dig that.