In April we're taking our two teenagers to the Southwest. We have time to visit only one one of the ancient Pubbloan sites. Would you recommend Chaco Canyon, Canyon de Chelly or Mesa Verde?

Good question, tough to answer. It's probably a tossup between Mesa Verde and Chaco Canyon, with a slight edge to Mesa Verde. Canyon de Chelly is wonderful, but if you have to choose just one, that would be the first one I'd eliminate. Mesa Verde is truly one of the great sites in the world. (It's one of the National Geographic Traveler Magazine's top 50.) Chaco Canyon is a relatively "new" attraction, and, still hasn't "developed" as a major tourist site, which is probably the best thing it has going for it and why I'd have a tough time eliminating it. The several park rangers I have talked with don't want to see it become as popular as Mesa Verde and would like to see it remain "relatively undiscovered," so that it could be preserved. Thus, and unless things have changed in the past year or so, you have to drive several miles (I think it's 20, but I could be wrong) over rough, unpaved roads to reach it.
You can't go wrong with either, but I lean towards Mesa Verde.
Of course, the best answer is: Go back again and again and see the sites you're missing this time!

I agree on Mesa Verde. The ranger staff does a great interpretive program and the ruins are easier to reach by road.
But, go early in the day. The tours to places like Balcony House fill up quickly. Cliff Palace is great also.
BUT, do some preparation before going.
It is a site where people lived. I find it beautiful scenery, but it should be more to the visitor than just some old mud huts stuck under an overhanging cliff. Who were these people? Where did they come from? Where did they go?
Why did they leave? How did they live?
All of these question stir the imagination if you have done a little homework before going.

I can't tell you about Chaco Canyon or Canyon de Chelly, but last Summer we went to Mese Verde with our three teenagers, and we all loved it. We spent a night at the Far View Lodge right in the park, which was very nice (if you do that, be warned - no TV's or phones in the rooms - but we did watch a coyote bound about in the field beneath our balcony - much better than watching TV!). If you go to Mesa Verde, I recommend that you take a ranger guided tour - we took the all day tour, which was great. We learned so much more than we would have just going to the sites on our own, about the history, culture, architecture, area plant and wild life, and more. Definatley worthwhile.

I'm afraid I'm basically another toss-up between Chaco and Mesa Verde - but I think I would lean toward Chaco Canyon. I like the fact that it's not as developed as Mesa Verde. I've been three times and have yet to see it anywhere close to crowded. I also happen to think that the 20 miles of bad road is part of its charm. If you do go to Chaco, take your own food and drinks - you can get water and a few snacks at the visitor's center, but that's all there is.