This is a very tough question to answer as all three members of Rush are amazingly talented.
Alex Lifeson is one of the most accomplished rock guitarists around today; he can play acoustic ballads or heavy rock anthems; his timing and rhythm are immaculate and live he is superb - some of the solos throughout his career is absolutely stunning.
Geddy Lee is an amazing bass guitarist who plays the bass more like a lead guitar. some of the riffs and bass lines he produces are pure genius. He is also an accomplished keyboard player and has shown his keyboarding talents off on many occasions. And of course he is the voice of Rush. Amazingly with the latest album he has shown us how much of an amazing artist he is; his voice is lower and he can no longer reach the screeches of the early days, but he somehow reaches those notes in a totally new and amazing way.
And then there is Neil Peart who arguably is technically the best drummer in the world. Just listen to his drums on songs like YYZ, Tom Sawyer and many of the newer songs and you'll realize that not many drummers out there could play those songs so well. He is constantly learning and in the new album shows that he is able to adapt and change his style. And then there's his words - Peart is an amazing writer and has written several books (and is collaborating on a novel based on Rush's latest album). However it's his lyrics that are astounding; they are deep, often retrospective and always complex.
Overall I'd probably end up saying Neil Peart, but it's a very close call!