The first one, Graceful Ghost Rag, might be the most famous piece composed by an American classical composer in the past fifty years. It is a great piece but I think it's best heard with the other two.
This performance is good in that it doesn't forget these are rags, after all. If I recall Bolcom took stylistically justified liberties in playing other peoples' rags, though I can't find that recording just now.
He's written a good number of rags. Here's another, Lost Lady Rag, played by John Murphy.
I would bet there are more piano students, alone, who have bought, borrowed or illegally copied the music for and played Graceful Ghost than have bought borrowed or stolen and played anything by Phillip Glass or Terry Riley. And that's just piano students. And there are transcriptions of Graceful Ghost for a number of different ensembles, as well. I believe Glass's most often played piece is the ensemble piece, Facade, from the relatively widely seen movie, Koyaanisqatsi, which might account for it being his most widely known piece, but I doubt most of the people who have heard it could tell you what it was called. I can't think of a piece by Riley which has anything like a widely known title. His most famous composition is "In C" which is less interesting than Kenneth Gaburo's "Flow of (U), neither of which is a serious contender as popular pieces, largely for the same reason.
I can guarantee you, any given year, you're more likely to hear someone playing Graceful Ghost in any music department's suite of practice rooms than anything from Glass or Riley.
I'd be very surprised if any serious commentator on classical music would hold that anything by either of those composers named in the comment is more well known. The closest thing I can think, in time to it, are some of the pieces by Aaron Copland, from more than 50 years ago, or some of those classical pieces by Leonard Bernstein. Arguably the dances from West Side Story (if you count those as classical, which I have no problem with) but they are older than that, too. I can't think of anything Bernstein wrote after the mid-60s which have retained their popularity. The quite beautiful Chichester Psalms might just squeak through on the time limit, but I doubt they are as widely known, either.
While popularity isn't the same thing as quality, I think Graceful Ghost wins on a combination of the two. And I won't make my joke about the modern convenience of "minimalist" music which both gives and fails the test of time on first hearing. OK, I just did. If it's the minimalist composers you want to talk about, I don't think any of them are better than Steve Reich, who is an entirely different level of composer, he writes interesting music that surpassed the "minimalist" label a long time ago. Perhaps that's who he meant, though I doubt any of his pieces has been more widely played and heard than Graceful Ghost.
In terms of quality of music, I think Bolcom comes out ahead. After the death of Eliot Carter, as far as I'm concerned, he's the dean of American composers, if someone so informal and democratic could be thought of in such a hierarchical manner. Off hand, I can't think of anything any of the others wrote that comes up to his solo music for piano, violin, other soloists, ensemble music, music for band, orchestra, voice, chorus and combinations of those. I think he's definitely the most skilled of all of the living American composers I'm aware of, and there are a lot of good ones who are relatively unknown. His friend, Curtis Curtis-Smith, for example. But they don't tend to write easily ignored, wallpaper music geared to find a mass audience, if people who don't actually listen to something can be an audience.
You don't diss William Bolcom to me and expect I won't answer it.
Update: So, I was right, he didn't know the difference between Terry Riley and Steve Reich. What he doesn't know about classical music is pretty much anything you wouldn't read off of liner notes.