Gonna add beatmaniaIIDX (Which I know was brought up in an earlier post) to my list, because the learning curve is the steepest I've ever seen for a rhythm game. The jump in difficulty between normal and hyper is ridiculous 
I suppose in general bemani games are fairly challenging, but I find Jubeat very fun (and pretty easy). Never tried DrumMania or any of those though.

I suppose in general bemani games are fairly challenging, but I find Jubeat very fun (and pretty easy). Never tried DrumMania or any of those though.