definitely the fly-half...

kicking is becoming more and more important in the modern game and a fly-half with a big boot can control the game by continually pushing the opponents deep into their own territory...

having said this his job is made much easier if his pack is on the front foot as this will buy him time and space which will have a positive effect on the quality and distance of his kicks...

also he can hardly kick successfully if his scrum-half is providing him with crisp ball regularly...

despite my earlier answer i have concluded that there is no single most important position and that rugby truly is the greatest team sport