This is a tough choice for me bc I completely agree with Sencha that what's most effective is what you'll actually do, and with toastedmoke that it depends on how we define effective.
In my own experience and those of people closest to me, I find walking the most effective for weight loss and maintaining a healthy weight. I'd love to move back to NYC someday, and a big part of its appeal to me is the walking-intensive lifestyle there. My brother lost over 100 pounds and has maintained it for 15 years. His primary exercise is walking (also cycles). I've also easily dropped pounds by consistently walking.
Although I think walking's phenomenal for weight, I find home videos most effective for reshaping and strengthening my body. I'd probably find weight training sans video similarly effective, but that's not really something I've ever been good about doing. I like the videos bc a trainer is right before showing me which strength training routines to do, how to have correct form, and how many to do. So for me, home videos provide the benefits of a personal trainer for a variety of workouts, including weight training, with the added flexibility of doing the workouts whenever I want to, plus I can pick different trainers depending on my mood w/o worrying about hurting their feelings.
So for the poll, I'm torn between walking and home videos, but ultimately I've gotta pick walking.