Which online forum will provide the best return on my time professionally?

It largely depends on (1) what type of work you do, and (2) where you're located. If you're a developer, StackExchange is definitely going to be more valuable to your career than either LinkedIn or Quora. If you're located in/near Silicon Valley and interested in or working with startups, I think I'd give Quora an edge over LinkedIn, just because of the connections that could be made here, however indirectly. Other than those two situations, though, I think LinkedIn wins out over the other two; I don't use Quora to further my career, I use it because it's fun and I get to share my expertise and opinions with others who value it - and do so in a fairly low noise:signal environment.

Create a "buyer persona" of what type of person would be most interested in what you're offering, and then research where those people are online. Since you're offering a B2B service, LinkedIn is likely your #1 choice, but there may also be large forums that cover such topics. Quora may also be good as long as your laser-target the specific topics that are most relevant and answer a lot of questions frequently to build your brand as a thought leader. However, over the long term, you definitely want your own website with a blog. A personal domain (not a free one) on an attractive website with calls to action and a form to capture e-mail addresses from prospects and leads. As you publish great content and promote it over social media, your "authority" in the eyes of search engines will increase over time, making it more likely that you'll rank highly for people searching Google for relevant terms. Today, any consultant or such person without an engaging website will not be taken that seriously. Good luck!