I'd have to say the ringed version. True, it was never used in actual scenes in an episode or film, but we know it's canon because its picture appeared on 1) the TMP rec room wall, 2) the 602 club in ENT's "First Flight", and 3) Admiral Marcus' office desk in STID.
Runner up goes to the Enterprise
-J as seen in "Azati Prime". Though in that case we can take some comfort in the fact that the "real" Ent-J will probably not look like that. Remember, the J came from an alternate future in which the Federation was involved in a long war with the Sphere Builders. Starship design was probably a bit "stunted" in that timeline because of the war draining all of Starfleet's resources (notice how the Ent-J looks a lot like a squashed NX-01).
That being said, I really don't understand all the NX-01 hate. Even if it is ugly, well, it's kinda SUPPOSED to be, ain't it? This is not exactly the pinnacle of technological advancement, the time period it's set in. I actually liked the NX, I thought it was nice looking. Even better with the secondary hull!