Im calling it now, and this is going to be unpopular and probably wrong, but I am going to say that Lumines is the best game on the PSP.
I am working my way through the pro tour on ridge racers, have played about 6 hours of AC!D, and a fair amount of Mingol (the other contenders) and I can honestly say that Lumines outperforms them all.
Here is the problem - I think alot of people dont get lumines. In fact, I know as much. Maybe this keeps it from being the best game on the PSP, because it requires you to get it at various levels before you can progress. However, it is exactly this reason that makes the game my favorite on PSP. For instance, take the impressions posted by Nash - they are simplistic and represent a level of understanding about the game that plateaus at about 70k (the I Hear the Music in my Soul skin, and the skin right after it)... once you truly understand lumines, it becomes something entirely different. The game rewards you for your understanding by presenting a remarkable musical diversity past skin 15 or so in single player mode. Reggae, drum and bass, jazz, flute... everything is represented and the gameplay molds itself into these forms. If I could explain how to get to 200k points easily (and I cant because it is a complex mix of understanding the immediate spatial relationships of a block, in terms of a large grid of color distribution for large scale chain combos when a special block comes along, and them combining this with an understanding of exactly how much time you have to execute before the scanline passes) then I think alot more people would be picking it over ridge racers. I dont think ridge racers is bad, in fact, before i hit 200k in lumines, ridge was my GOTY on PSP.... but lumines changes so much, shows you so much more at the higher levels... it is impossible to resist. Im just completely impressed by the game the further I get.
One of the strangest things about it is that I seem to do better on skins where I really love the music... which means that, at least for me, the mix of music and puzzle works on different, more cerebral levels.