What are some more harmonious aspects in regards to compatibility?
If we look at it based on the signs only, then the aspects look like this:
1. conjunction - same sign (same element, same mode, same gender)
2. semi-sextile - one sign apart (different element, different mode, different gender)
3. sextile - two signs apart (different element, different mode, same gender)
4. square - three signs apart (different element, same mode, different gender)
5. trine - four signs apart (same element, different mode, same gender)
6. quincunx/inconjunct - five signs apart (different element, different mode, different gender)
7. opposition - six signs apart (different element, same mode, same gender)
So obviously semi-sextile and quincunx/inconjunct have the least in common, even square and opposition have more in common.
However, if an aspect is good or bad in the end depends on the entire chart and what you have in mind for a particular relationship. If you want a dynamic and exciting relationship, then some hard aspects are more or less necessary. If you want a quiet relationship then more soft aspects are necessary.
Also, you have to look at what houses will be activated because sometimes there's a trine (which is generally seen as 'good') but if it activates your 6th or 12th (which is generally seen as 'bad') it will leave quite a different impression. What you also have to factor in is the level of maturity on both sides or age gaps. If maturity is totally lacking, then the best aspects won't help, if maturity is there to a very high degree, then even the worst aspects won't do much harm and may even be welcome. Relationships are always a very complex matter and that's why there aren't really any hard and fast rules that would apply to just any relationship. It's quite fascinating though.