I've done both 90X and Insanity. Haven't done Asylum.
But what you're really asking is do you go cardio or do you lift weights? Because these programs are meant to work you in very different ways. Insanity is all about the cardio. P90X is all about muscle tone with some cardio thrown in.
Personally, I'd go (and went!) with P90X and it's not even close. Insanity has a major dread factor with me and all the workouts are really similar. They're not fun, painful and didn't produce any visible body or weight results. I got major cardio endurance gains, but that was it. I get great results with P90X from both the bathroom scale and with muscle tone.
FYI, Kenpo is probably the easiest workout of the bunch.
PS - If you're looking for cardio only and you want "fun", I'd probably recommend Turbo Jam/Turbo Fire. But I still vastly prefer P90X.