What's the best way to get from San Francisco Airport to Oakland Airport?

The Bart train.

I believe that someone (maybe Airport Xpress) runs shuttles between them—BART will be fine, but will take maybe 90 minutes. The shuttle will take around 45 unless you’re trying to do it in really bad traffic.

Ask a friend to drive you.

It depends on your time and $ constraints.

Without additional info, I would suggest booking a flight from SFO to HNL and a return flight from HNL to OAK about two weeks later. Of course, I always recommend another interisland flight to Kauai while you’re there!

If this doesnt fit your budget, there are certainly cheaper routes…BART included… but “best” is a subjective term ;)