Whatever works best for you, of course, but I'll give my opinion
MSAA is still great despite newer forms of AA claiming to be taking the title of greatest AA ever. It provides smooth edges on geometry both near and far. It's only flaw, other than its moderate performance hit, is that it is applied before many shader effects, including those with heavy use of alpha channels, which Skyrim has a lot of. Also, it has no effect on the alpha edges of transparent textures; Grass, tree leaves, etc., but that has never bothered me.
I used SMAA for a short while, and I can say I liked it. Unfortunately, it uses a d3d9.dll file which interferes with some other features I like to use.
There's something funky about Skyrim's FXAA. For me, and many others, it undoubtedly has this overpowering blurring effect. Maybe not a bad thing for a lot of people, but it bothered me. Happily, from what I've seen, I can say nVidia's driver forced FXAA gives much tighter image quality than Skyrim's FXAA, closing in on the geometry smoothing of 4x MSAA while also effecting things like alpha channels.
I think nVidia's FXAA mixed with 4x or 2x MSAA would look fantastic, but my PC has limited horsepower. I stick with nVidia FXAA alone.