I like all of them. I just wish 1 and 2 were on DVD.
Powerstrike has a very 'authentic kickboxing' feel to it, and the form is excellent. If that's what you are looking for, they are perfect. If you want a 'dancy, cardio workout with some kickbox moves," they aren't the ticket.
As Laura said, 1 and 2 are more intense, but I like 3 as well. It builds up more gradually, and allows you to really focus on form. If you hit hard and go low on squats, it's an intense workout as well. I think it feels less intense to some because of the more gradual build-up. But I sweat like mad when I do it (I do add weighted gloves, though, which I've been doing with most of my kickboxing workout for several years).
And if you can find any pre-millenium Powerstrike workouts, they are all excellent, and all high intensity. There are about a dozen in all, including the Milleniums, and including a bag workout and a forza (sword fighting) workout, and a 'kicks only" workout. The farther back you go, the worse the production value is (their very first workout almost makes me nauseated if I watch the screen too much, because it looks like it's been filmed by a 12-year-old with an old camera without motion stabilization. With any other workout, I would have gotten rid of it long ago, but the workout itself is so great, it makes up for even the most horrible production values, IMO).
Illaria (one of the two instructors, the one with the martial arts black belt(s)) is coming out with two new workouts sometime in July (latest update). One Powerstrike kickboxing and one Forza (sword). There are also future workouts in the plans. These are workouts I have a hard time waiting for! I need more Powerstrike!