I have all of Margaret's DVDs except Maggie. Margaret is one of my "go to" instructors -- she's so quirky!
I like all of her DVDs, some more than others. I just finished an Unplugged rotation (both 1900 and 2000) and liked those. My impression is that Margaret's workouts have gotten easier over the years, but I'm not sure why I think that? Neither the 1900 or 2000 Unpluggeds are killer, but that's partly because of the 25 minute format, which includes a short warmup, a short stretch, and about a minute of Margaret's thoughts on life and exercise (called "Food for Thought") (so the workouts themselves really are 15-20 minutes).
It's been a long time since I've reached for some of her other workouts, but I'll echo the recommendations for Triple Threat, All My Best (both Vols), and Sculpture. The one I reach for most often is Good for You, but I think this one is on the easier side (which is probably why I like it!).
I don't find Margaret's stretches very effective (for me -- others might), so I wouldn't start a Margaret collection with Muscle and Grace or Strong and Limber, since half the DVD is short stretch workouts. Just a suggestion...
My recommendation would be to either pick a DVD that has workouts of a length that you like (she has 20, 25, 30, and 60 minute formats) OR start with All My Best (either Volume) to see which ones appeal more to you. You could also read some of the reviews. I think there is at least one review of each of her workouts in the VF database.
Hope that helps!