Hi
@shacha, welcome to the forum.
In my experience, with an active career like Detective, the 'best' aspiration is one you will enjoy completing. Friend of the World will get your sim meeting others and you can learn their traits through socials so that could be useful. If you are planning for your sim to have a family then maybe Soulmate would help. Focused is the emotion most useful for Detective so learning logic would be good and that matches in well with a couple of the knowledge aspirations. Remember you can switch around the aspirations without losing progress, so you could do a bit of Bodybuilder, switch to Painter Extraordinaire and then go back to Bodybuilder if it suits your gameplay.
Sometimes it helps to pick a particular aspiration in CAS just for the bonus trait even if you're not really fussed about finishing it. So some people pick a knowledge aspiration in CAS because the bonus trait speeds up all skilling even though they plan to focus on Master Chef or Angling Ace.
Most important of all though, have fun
