I still think EZ2DJ is the best DJ simulator out there.
O2Jam is a blatant rip-off and would be popular because of the character customisation. The songs are overall fun but it's just a shame that most of the good stuff needs to be paid for to be unlocked. And, yes, I understand that they are running a business and this is the perfect business model.
IIDX is the originator but its genre range is limited and is more for the expert players -- especially with the difficulty curve it has. By "limited genre range" I mean that most of it is too trancey now. Sure it sounds fantastic but even so, the song range is very hit'n'miss for me... Trance to IIDX is like Naoki to DDR :P The difficulty curve is... well, you really need dedication to perform well on it -- something that the average gamer just doesn't want, I suppose. And because it *is* so hard, it's very easy to become frustrated with the game and quit altogether. On top of that, everyone knows that it stole songs from DDR!!!11
EZ2DJ, on the other hand, has a tremendously fantastic music range, fantastic background videos (call me old-fashioned, but the IIDX videos look like absolute shit compared to *any* other DJ simulator's videos) and the notes are fantastic -- you *don't have* to play with the 2 Effectors + pedal... You can toggle them. The hold notes are a fantastic innovation. The videos are fantastic quality. The timing is great -- not too hard, not too soft. The notes themselves are pretty decent for its time (of course there's always room for improvement but that goes with all games)...
It's just a shame that Konami killed it
Oh well, at least DJ Max lives on... I just don't like the limited song count... but you can't do too much with a UMD's storage capacity... Otherwise, it takes what EZ2DJ had and can still manage to improve on quality. The genre range is fantastic and the songs are enjoyable to play on all difficulties (well, most of them are... Some are just retardedly difficult). The videos are much smoother than even EZ2DJ which blows my mind. Again, it's major downfall is that there are only some 40-odd songs. And for the amount of time it takes to unlock everything... well, you get really damn tired of those songs really damn fast. Although, having no turntable (for the most part) would increase the average English-speaking gamer's interest.
So, in summation: O2Jam should dominate in sales. DJ Max a close second because there's no turntable but the song count isn't strong enough. EZ2DJ would be third ONLY BECAUSE its genre range would better suit the average English-speaker and because of this, IIDX would be last (hey, beatmania US pretty much failed, didn't it?). All too many times have I heard people tell me off for liking trance. "What the hell is this? This isn't music!" but they'll appreciate hip-hop and r'n'b (which the Korean knock-offs are pretty much famous for).