Well, the newest one obviously, but all of them are really good programs. For example I have been using Photoshop CS for the quite a while now, I only recently purchased CS6 and for me there is not much of a difference in what I use, I just have more filters, that's all.
I know someone here mentioned getting Elements, I wasn't too thrilled about that software myself, I had a lot of errors and crushing thoughout using it. Maybe the newer version of it fixed it, the version I was using was Elements 2.0.
I'm not a professional if it come to this software though, I only speak form experience. Don't throw too much money without testing it first, I would suggest to download a 30 day trail version through their website and see if it suits your needs, this is an expensive product.