From what I've read, GnU was pointless and just...weird. Has that been released to the states, though?
Alcor was very cute. It's more about a few side characters doing side things before the main storyline for G.U. kicks in. I wouldn't suggest reading it if you're looking for something in that timeline, but I would suggest it if you're in the mood for a cute story with cute art to match.
Cell was both a very interesting and satisfying read. I enjoyed it very much, so I won't say much about it so as to not spoil anything.
I like GnU. It's about kind of an F Troop of Moon Tree, what's not to love? It's different from most at least in that for the most part it's just a story about totally normal players, so I think it's worth reading, though I haven't finished since G.U. The World has been, surprisingly, harder to collect than normal The World (which I somehow have two sets of now)