I have quite a few on my list.
Newton and Leibniz since the day I learned they were 22/19 (respectively) when they invented the calculus, Riemann as well (one of my teaching assistants was mad about him as well... it caught on)
Gödel after I'd took a course focusing on completeness and incompleteness, as well after you read his biographies.
Saharon Shelah, after one of my professors that did his Ph.D. under Shelah told me a lot about him. Finished his master degree in one year, Ph.D. in two. Invented so much... he's a real inspiration for me.
Grothendieck is a personal inspiration from another end. Not as a mathematician but as a human being. The fact he was able to get up and leave everything. That is amazing for me.
And while we're at it, Albert Einstein since I was 21 and read the book Ideas and Opinions.
What matters is less the work, but rather the ability to express with clarity a new idea that no one had before. That's what makes a great mathematician in my eyes... at least from where I stand today.