Rambutan

Shaking shadewood trees is the only real source and the odds are poor, so this is something you collect while clearing a Crimson forest rather than something you set out to farm. Shaking costs nothing: hit the trunk without felling the tree and it drops whatever it happens to be holding, then goes on a cooldown.

Eaten raw it gives the weakest of the food buffs, which is fine while you are exploring and not worth the inventory space before a boss. The better use is shimmer. It feeds into Ambrosia, which lands at the top of that same buff line, and that is a real upgrade out of a fruit you got for free off a tree.