It would really depend on what your goals are. If strength, get the Pyramids. If cardio, get Imax2.
If you don't care, get Imax2 first, then the Pyramids. I have yet to do Imax3, even though I've had it since it was released. I know that someday I will do it, but it was just too advanced for me, and the choreography too fast and intricate for my fitness level. I'm sure it will be a blast when I am finally able to do it, but I thoroughly enjoy Imax2 right now, so it's my top recommendation. The music is fun (even today) and the choreography is interesting enough to never get dull, yet basic enough that I don't trip over myself. And it's a sweat-fest!