Chen style Taiji comes first, historically speaking. From that came at least two variations of Yang style. Wu style derived from Yang style. Wu (Hao) derived from Yang and Chen style. You can find more details of the actual lineages on the web.
Personally, if you're just interested in the "health" aspects of Taiji, then any of them will do just fine. All are relatively easy on the body, with slow, relaxed movements. Each will improve flexibility, balance, blood circulation, coordination, and functional strength. Chen style will also incorporate fast movement in the second long form you learn (Pao Chui), which makes it more athletic than the others, and generally more martial in my opinion.
Yang is rather elongated, has deep stances, and makes large, overarching circles with the arms with smooth motions that appear to have no beginning or ending. Wu style, on the other hand, is abbreviated, and movements are small and more linear. Chen style is also large, like Yang style, with much more storing and releasing of power (the result of coiling into / around joints), and variations of tempo rather than having smooth, continuous movement throughout.
Usually the question asked the most is which one is the most martial. That would generally be answered with most people saying Chen style. Chen style doesn't hide its martial qualities, and in fact everything in Chen style is for martial arts, not health, though you certainly will improve health doing Chen style.
Yang and Wu style don't exactly "hide" their martial qualities. It's just that it's harder to see. As a result, most of the people teaching those styles don't really know how to use it martially. (Or they think they do, but actually have ideas that are often quite wrong.) So they concentrate on the health and meditation aspects.
So if you're just interested in the health and meditative aspects, you should probably stick with Yang style. It has the most crowd momentum behind it for that purpose. It's also the most popular of all the Taiji styles. Following that is Wu style close behind. I often see Wu style being taught to the elderly, because of its shorter stances and motions.
However, if you have any interest in the martial side of things, or if you want a more physically challenging experience, take Chen style. It will have all of the health and meditative aspects that the other styles have, but you will also spend time working on the martial qualities, which can be very interesting. It will also be more physically demanding, in my opinion.
By the way, just to note. What I said above is mostly just generality. Not all Yang style people have no clue about the martial side of Taiji. And just because someone is teaching Chen style doesn't mean they're superior to everyone who teaches Wu style. You have to do some research, ask around in your town who knows what. And go see all of the classes. My personal preference would to find a teacher who knows what they're doing, regardless of style, instead of looking for a particular style of Taiji.
Look at some Youtube videos, also. You can get the gist of all the different types of Taiji from watching the way they do their forms.
Hope that helps.