I would say that honestly, most introverts really intrigue me (yeah all of them!). And also extroverts who appear like introverts (ENTps, ENFps...and then I'm SO sad when I find out their type .... Of course, this all crashes and burns later on...*wearing shirt that reads " still haven't met my dual" lol) Anyway...

For romance:

1) I still think duality is best for romance. Dual friendships I think could be good, but I haven't known one that I've been best friends with yet.
2) Activity is also good!!! warm fuzzy-ness
3) Semi-dual is also very good. also warm fuzzy-ness
4) supervision is very interesting, especially at first, and though there is some conflict, it's not all that bad
5) Look-a-like -- not bad! but they must be really interesting as a person for it to work long-term! They lack the "Mystery" factor so must be super cool for it to last.
Note: I don't like identical for romance because there ends up being a lot of either 1) insecurity because you assume they have the same tricks up their sleeve you do 2) annoyance or boredom if you realize they are exactly like you

For friends:

1) Identical all the way! Just make sure you get a non-boring and smart version like yourself...few things are worse than a boring identical I have an identical friend and we say we are clones...we react exactly the same way (obvious, but still surprising every time). Tonight we were both at a movie and both were overly concerned that we find a drinking fountain and bathroom before the show started - I mean, no one else in the group cared...but we have those Si concerns lol, and then we bitched about small Fi things people had done wrong that had upset us, that no one else thought was important, etc.
2) Activity!
3) Semi Dual
4) Mirrors I get "in theory" but I've had one good INFj friends, and honestly she annoyed me (but I think it was just her). She would act like everyone needed to think the same way she did, as far as supporting the exact same humanitarian causes (and sometimes I didn't want to buy a magnet for whatever, etc.) There were some things I really liked too, but in the end I was too annoyed by certain things. I'm sure most INFjs aren't like that though.

For friends you go out w/ and don't talk deeply with, or see a lot:

1) ALL types!! Even conflictors. I've had several ISTj girl friends who I really do admire and like, and because we're not roommates or anything we never have conflicts (mostly because I avoid saying things I know would set them off. I'm always aware that if I say certain things, especially Ne things, it will cause a conflict.

The worst imho: comparative! conflicting, quasi-identical, and illusionary is not all that great either.