Which cocktail has the highest ratio of "tastiness" vs "effort to make", assuming you have to be the one to make it?
Neat whisky? If "cocktail" requires some combination of ingredients, cask-strength whisky with a few drops of water. If you're going to be a stickler for something that is conventionally referred to as a "cocktail", there's a ton of stuff you can just pour into soda water and end up with something delicious (I've been on a Sambuca Soda kick lately, inasmuch as I ever drink anything these days).
(The fundamental issue with this question is that it assumes that effort is correlated to tastiness, which just isn't true—at least for my palate. Higher-effort cocktails are valuable because they afford novelty, fanciness, and can better target the current mood, but there's extremely tasty stuff at every effort level.)