Basically, the reply to your question is a matter of tastes, and some will say it is this or that the best option, but as you know, when tastes come into play, it's hard to say which one is best. Each and every one of them has tasty things...
My favorite flavor in this case is Onealex because it has updated campaign files, lots of ships included in the roster, nice radars in the sub and a looong etc.
However, GWR has nice things too, like updated Italian ships, (I think it has the same subs models as Onealex, I am not sure)...
So far, I didn't test the new release of the GWX mega mod, but Onealex has all its features included... The best thing you can do is try them and see which ones suites your tastes... Hope this helps.
