krav maga?

It's decent exercise, but pretty physically intense in the sense that you get kicked in the chest a lot. There's a bit more of a philosophy of "you need to feel the pain" than karate or jiu-jitsu.

Lots of punching drills, which is good for back/arm/shoulder strength, and it definitely keeps you moving. In terms of pure cardio, kickboxing is probably the best bet, but krav maga is way more fun, and definitely one of the best practical systems for self defence.

Depending on the drill, Krav can be a great workout. Of greater benefit, though, are the skills you learn. As someone mentioned, it is VERY practically oriented and teaches skills that could potentially save your life. Part of the philosophy is to tire you out physically at the beginning so that you experience something similar to the adrenaline dump you would experience in a moment of panic as when faced by an attacker. We work on scenarios such as dealing with multiple attackers, since they do not fight fair. In a given class, you might work on basic combatives, such as different types of strikes, such as elbows, straight punches, upper cuts and such. You also work on how to defend against knife attacks (assuming you can't just run away from the situation to begin with) and the myriad of ways someone could attack you with one. You also learn to defend yourself in situations where someone has other weapons such as a pistol to you. In today's world, I could not recommend this more strongly.