What are the best aspects/placements in composite chart?

Here are some of my favorite aspects in composite charts.

Sun and Moon in good aspect (sextile and trine).
Moon and Moon in good aspect is important for emotional compatibility.
Venus (love) conjunct, sextile, or trine Mars (sexual desire).
Venus in the 5th (romance) or 7th House (marriage).
Venus trine Saturn (personal responsibility).
Mercury in good aspect to all the planets.
Venus conjunct Ascendant (also called Rising Sign).
Venus conjunct Descendant (7th House cusp)

I think that the easy aspects of sextiles and trines (and some conjunctions) are karmic rewards from something done especially well together in a past life(lives). The easy aspects make for a comfortable compatibility where 'awkward silences' don't exist.

The tough aspects of most conjunctions, squares, and oppositions are what need to be worked on in this lifetime together. Mars and Pluto in hard aspect is a really tough aspect to overcome but as with all the hard aspects, cooperation and compromise can greatly improve and hopefully eventually overcome the tough aspects. A chart with no hard aspects can actually be quite boring so a few life lesson aspects keep things interesting.

Hope this is helpful and good luck and bless you too.