Biden. I would randomly start slurring my speech and "pass out" at a campaign rally.
It would more or less take him out of the race and make it probable that either Warren or Harris, both of which are considerably weaker candidates, would be nominated.
Good choice and smart strategy.
If I were in Biden's body I'd just say my real controversial beliefs out loud and tweet them as well (not going to elaborate for fear of getting modded).
If I were in Bernie and Warren's body I'd have fun by saying the most radically far left things I could think of and seeing how far I can get before my fans stop defending me.
When it comes to Harris and Buttigieg...well, let's just say I'd get extra 'creative' if you catch my drift. Same with Yang.