The easiest finish is either a pure oil, or else some kind of friction polish.
In terms of safety - that question is likely to draw some debate. I believe that any finish is 'food safe' once it fully cures. That said, I would be hesitant to use a shellac-based friction polish on a stopper that will be used with wine because alcohol can soften the shellac. I don't think there would be any problem with a lacquer-based friction polish.
I use two applications of Tung oil on stoppers that have a captive ring - the first diluted 1:1 in turpentine, and the second full strength. The main argument is that this is fast and can be applied on the floating ring without concern for 'gluing' it to the body of the stopper. On stoppers that are solid turnings, my preference is for gloss WOP. Fully cured WOP is food safe and alcohol resistant, and it also looks pretty nice. But it takes longer to apply the 4-6 coats that I prefer.