Relevant:
According to SimilarMinds (I'm not sure why or how they've come to this conclusion), the ideal romantic partner for each type is:
1w9 x 9w1
1w2 x 9w8
2w1 x 8w9
2w3 x 8w7
and so on.
I feel a strong romantic attraction to 4s, especially 4w3s, but they tend to make poor friends for me as the attraction stems not from commonalities, but from each party having something the other doesn't (and wants). If you consider my Enneatype (6w7) this fits the theory. While this is only an anecdote, I struggle to understand 7s, while 5s and I are simply like oil and water.