Become a Sims 3 "Tiger Mother"
I'm playing a super-hard challenge these days, which requires (among other things) that your Sim max out their career AND a skill. I have traditionally had a lot of trouble getting either of these things accomplished, much less in the same Sim. It has really brought home to me the value of matching up skills with traits and career.
In this sense, the "best" skills are the ones that Sims can start building in childhood. Toddlers can learn some musical ability by playing with the xylophone, and can learn Handiness by playing with the blocks toy. Children can fish, read books (which helps improve your ability as a writer later on), paint, play chess and use the telescope for Logic points, and use the toy oven for Cooking.
If you have enough forethought, you can choose a career while your Sim is still in swaddling clothes. This lets you make the appropriate choices for them, in order to maximize their chances for success. (An approach that typically backfires in real life, exceptions like Tiger Woods not withstanding.)
Each career relies heavily on at least one skill for your promotion. According to Carl's Sims 3 Guide, these are:
- Criminal (Evil Branch): Athletic, Logic
- Criminal (Thief Branch): Athletic
- Culinary: Cooking
- Journalism: Writing, Charisma
- Law Enforcement (Forensics Branch): Painting, Logic
- Law Enforcement (Special Agent Branch): Athletic, Logic
- Medical: Logic
- Music (Symphony Branch): Guitar, Logic
- Music (Rock Branch): Guitar
- Military: Athletic, Handiness
- Politics: Charisma
- Sports: Athletic
- Science: Gardening, Handiness, Fishing
As you can see, if you're not sure which career you want for your Sim, then Logic and Athletic are the best skills to focus on. Both of these skills are crucial for five careers. I believe Military is the only career you can start practicing for as a toddler, with the Handiness skill from the blocks toy. Start your astronauts young! (The Music careers both require the Guitar skill specifically, not the generalized Music skill).
In fact, if you really wanted to Tiger Mother your little Sim from the beginning, you could start with the pregnancy. If you do a good job at keeping the mother happy, fulfilling her wishes, reading pregnancy books and so forth, you will be allowed to choose traits for your baby Sim at birth. For an astronaut, you would want to choose Handy and Athletic, since these traits both correspond to better success with their respective traits.
Each skill has a trait which directly affects your Sim's ability to learn and perform that skill. It really does make a significant difference in how quickly your Sim learns that skill. To such an extent that it's worth it to fulfill a lot of your Sim's wishes in order to get the "Mid-Life Crisis" Lifetime Reward so that you can re-choose their traits, if you don't end up with the traits you want. (Maybe you didn't raise your Sim well, or maybe you chose a trait before you started planning for your Sim's future. I don't judge! And I have made both mistakes myself, many times!)