It's actually too hard to decide. If Zuko didn't teach Aang firebending, he wouldn't have benefitted from the comet or be able to redirect lightning. Toph vision actually helped Aang when Ozai was gonna burn Aang after sparing him, and of course the lion turtle was the one who taught him energy bending....

For me its got to be his airbending skills he learnt from the monks, in nearly all his fights he uses evasive skills to avoid getting hurt especially against ozai where he was jumping from rock to rock he would'nt have been able to defeat ozai without it

in "paper" the turtle seems to be most important

but just like a painting, one of the colors might have been used the most, but if we left only that one, and took away the rest, the painting will not only lose its "personality", it will stop being a painting, it will be just a canvas colored to a certain color

so all masters end up being equally important to the overall character of Aang at the end

and master is not always having to relate to bending, it can be also to the mind

so besides the mentioned Guru and Roku we have other masters who influenced Aang in bending and life teachings

Jeong Jeong, pakku, Iroh and the chief of the sun warriors, maybe even Wu

Well, Jeong Jeong didn't teach Aang, Aang kind of screwed up and learned the hard way.

even Sokka even if he had no bending powers can be viewed as essential to Aang at the end of why Aang makes his decision with Ozai

If it was which Master had the most influence, I would say the Monks. The most beneficial? That is much harder to answer. What the lion turtle taught him was beneficial. Had Aang not learned that, he would have had to kill Ozai.