P.S. As far as waza is concerned, I can only narrow it down to three: ikkyo, irimi-nage, shiho-nage.
P.P.S. O'Sensei said that there are 6,000 basic techniques in aikido, and 11 variations of each of those techniques. I never heard him name the most important one, but I've heard it said that he had students work a lot on irimi-nage -- perhaps because learning to enter is at first counter-intuitive and therefore needs more time to learn.