Whatever fits! But mostly the ones I find to be more effective for gameplay; some of the traits are just jokes, intentional or not!
So almost always charismatic and friendly because they really further any social interactions, with highly skilled charisma of course, and ambitious for the extra LTR-points and 'over-emotional' for the great enhancement of any mood, that leads to the same; then traits that further any plan for the sims life, favourites being artistic, bookworm, green thumb; and the two extra from UL I use for fun, cute and weird choices like 'brooding' for which all the idles are adorable, 'clumsy' for the cute way they trip and then look around them, 'excitable' and 'loves the outdoors' for the moodlets as well as idles - they "woo!" and enjoy the rain! My new favorite is 'unstable', mainly not having it figured out yet.
I never randomise, those aspects are randomised enough in the game in my opinion, but constantly change traits with the Brain Enhancing Machine (UL).
Once, playing the Pertridges in Hidden Springs, I played 'evil' and 'mean-spirited'-sims - but have never picked those traits myself, that kind of crude gameplay bores me, especially when the sims do things autonomously; 'irresistible' and 'flirty' creates so much more drama! If 'inappropriate' did not lead to autonomously being mean I would pick it more often - great trait in my own completely impartial opinion..
