Most similar...I don't see these people on the board much now, except for Jennifer (even though I often think of her as an INFP sometimes). Tallulah, INTPness, Scorpio-something-or-rather. I can't think of too many INTPs right now. Similar to some of the XNFPs here. It's probably because of the overlap between the 5w4 and 4w5.
Most dissimilar...Ism, Pseudo off the top of my head. Again, I think this is enneagram related.