It probably depends on how they relate to you (as a friend, partner, relative...).
But on average, what stacking appeals to you the most in another person, in contrast to the other variant combinations?
Perhaps you also noticed that a specific enneagram type comes with it, mention this as well.
To me it's either SO/SX or SP/SX. Fellow SX firsts - too intense. Ideally, it's like how I relate to @niffer but in general, secondary SX is better for me and them somehow. The SX lasts - not intimate enough, respectively. If I had to decide between the two of them, heart triad SO/SX wins by slight margin. My SP/SX favorites usually coincide with head triad types. Maybe SO/SP makes the top three complete, I get on well with them and find what they say sympathetic in many cases.