I can tell you absolutlely, positively that there is no absolute right answer to your question. Most people would say green tea is the healthiest...it would be hard to argue with that. However, recent studies have shown that green tea is not alone. Black, white, pu-erh, and even oolong have received positive results.
So, since they are all from the same species of plant, Cammelia sinensis (although there are many different clones), I would say let your palate be your guide. Everyone has their own personal favorite...mine is green...but I also like oolong, white, black, and pu-erh in that order (the order is always subject to change).
There is so much info on all these teas on this forum, I would do a little reading and ask questions when you need to. You will need to try some fresh looseleaf tea from reputable vendors...however this will make it almost impossible for you to go back to supermarket bagged tea...just a warning. But you will never miss it since there are seemingly endless looseleaf teas to try, and you will be hard pressed to pick a favorite.