I've only been doing yoga since January, but I love it.
Namaste Yoga (the old FitTV show) is my relaxing favorite. There's 12-13 22-minute episodes in each season, and the last 2 or 3 are pretty challenging. I can do it in the evening and hop right in bed.
For a bigger challenge, I like The Biggest Loser yoga DVD. I'm still working on the level 1, which is 30 minutes total. I'm sweaty and trembly-armed when I'm done, though not as bad as when I started it in February. There's quite a bit of holding plank in the middle.
Lost the first 100 with Medifast, now working on the last 25