I agree with Sabra that the best exercise is one that you will DO. I could have a mountain of Tae Bo dvd's at my house-and they wouldn't do a nit of good-because I would never, ever do them no matter how good of a workout it is.
Any exercise is great while nursing-that doesn't inhibit what you can, or cannot do.
I dance, walk, do yoga, and strength train, currently.
Also-I think the amount of time you exercise should start small, and then slowly increase. You want it to be a part of your regular routine. Don't start exercising for 2 hours a day like a lot of people do at New Years, when they make their resolutions. They tend to start off this way, and burn out a month later and quit. When I first started exercising regularly 8 years ago-I started with 10-15 minutes a day when my oldest daughter was napping. A couple months later I was averaging 30 minutes. Now it is anywhere from 45 minutes-6 hours a day (occasional out of town dance workshops!) depending on the schedule. But-it is consistent.