sp/sx with sp/so - their aims coincide in themes of primary self-preservation instinct, but he (sp/sx) wants to spend more time alone while she (sp/so) wants to go out and socialize, she will occasionally chide him for being so insular
sx/so and sx/sp - same issue as above, sx/so person wants to go out more while sx/sp person doesn't feel the need to
sp/so and so/sx - she (so/sx) occasionally complains that he is too conservative, which may be related to their mis-match in self-pres but overall I'd say they go well together and she makes up for his lack of sx
sx/sp with so/sx - same issue of him (so/sx) being more social and outgoing than she would like to be

I've re-evaluated the stacking thing recently, happened kind of accidentally after a dream, and my pervasive feeling is that the worst pairing is where your stackings are inverted i.e. if you're so/sx then the worst match is sx/so, if you're sx/sp then the worst match is sp/sx and so on. The reverse instinct flow of the other person will be actively extinguishing your own primary link. In real life what this amounts to is that what the other person stands for is just so completely antithetical to your own being that this is by far the worst match. There's this subtly adverse dynamic that is created in such cases.

The next worst but-slightly-less-so match-up is the stacking that shares the same creative (secondary) instinct but different primary instincts: for so/sx this is sp/sx, for sx/sp this is so/sp, etc. In this case you aren't extinguishing each other's primary link, only the secondary one, which is less grating. However, I have a feeling that both people kind of slightly look down on each other because neither appreciates the other's primary focus, but the shared middle instinct gives some common ground.

A slightly better match out of the opposite flow is the one where you share the primary instinct but differ in the creative instincts: for so/sx this is so/sp, for sx/sp this is sx/so. In this case you don't extinguish any links of the other person. There is also mutual appreciation and understanding of each other's primary focus since the primary instinct is same, unlike in previous case. The main problem is that the other person may seem somewhat boring, conversation may be difficult to start, interests may not coincide, since the secondary instinct usually outlines what conversations topics and hobbies people gravitate towards in order to achieve the needs of their secondary instinct. It's like you're both trying to achieve the same thing but tapping into different areas of life to get it. In the end you may just part and go your separate ways, but out of the opposite flows I feel that this is the best match-up.

Within the same flow the best match is where instincts completely coincide. The other matches within the same flow create something akin to socionics benefit relations, where there is mutual attraction, but one partner slightly undervalues the other. I think this happens because one of the stacking reinforces the other's second link, but receives no reinforcement back. Thus the dynamic is asymmetric, but it still better than matches with stackings of opposite flow.

so then for sp/sx matches from best to worst would be: 1) sp/sx 2) so/sp or sx/so 3) sp/so 4) so/sx 5) sx/sp
and for sx/sp they would be: 1) sx/sp 2) sp/so or so/sx 3) sx/so 4) so/sp 5) sp/sx