I think that Java moss is the easiest to grow, but it also depends on what type of animals you have. If you have herbivorous fish, snails, etc, you might want to stick with Java Moss since it has a bad taste and they usually won't eat it.
What are you going to use to secure the moss? I've been looking into making a moss wall myself (48x28 on my 70 gal), but I can't find anything in local stores and only really expensive stuff on ebay (which I have to buy about a dozen of since I have such a large tank). I've been trying to find something on other aquarium suppllies websites, but so far I haven't had any luck.