The first trait I buy for all of my Sims is actually Observant. It saves a lot of time getting to know other Sims. Of course, aliens and vampires have their own ways to figure this info out but for everyone else Observant is a must. Gym rat for active Sims is also a good one and Carefree and Shameless are just about indispensable for me as I really hate the Tense and Embarrassed emotions. So these two are on the top of my list when it comes to more expensive traits. After that it depends on the Sim I'm playing with and what they need/goes with their personality.
