There are lots of ways to make money through skills: writing, painting, playing guitar for tips, breaking space rocks on the board breaker - I think photography as well, but haven't tried that one. I think it is possible to make $50,000 or more with any of these at a high level of skill. As to which one is best, I think that depends on the traits you give your sim as well as your personal preference.
Edit: I forgot to add, Carl's Sims 3 guide
has a lot of useful information on all these skills and how to use them to get rich.[Edited to add a link to the guide. -Pam]