I'd go with two franchises.
beatmania IIDX and Taiko no Tatsujin.
These two are two great music games franchises from Japan. Both of them got one game in America and Europe, but they commercially failed in these continents, while they are still very popular in Japan.
Both series have a lot of games, on both arcade and consoles. beatmania IIDX has 15 PS2 games while Taiko no Tatsujin has 11.
beatmania IIDX, from the creators of Dance Dance Revolution, is
supposed to be a DJ simulation game. You play with
that kind of controller. The goal is to press the keys and to spin the disc at the right moment to "create" the music you're playing. Here's a video of
a moderately hard tune, and here's a video of
a tune that is so hard, that you'll have to give up your life to win it, especially on the right side.
Taiko no Tatsujin is a Taiko drumming simulation game. You play with
these drum controllers. The goal is to hit on the drum or on its edge at the right moment to drum over music. Here's a video of
a medium/hard tune, and here's a video of
a tune that is, once again, freaking hardcore, especially after 42 seconds.
I found both of these series pretty addictive. I especially like IIDX's soundtrack.
A downside for these games, is that they are region-locked to Japan, so you'll need either a japanese PS2 or a modded one. Unless you find an out-of-Japan version of these games, if you're lucky. But to be honest, in term of challenge and song selection, I prefer the Japanese versions.
That's all I had to say about these games. :)
