I think I'll stop here. - Andrew Wiles

But the best problem I ever found, I found in my local public library. I was just browsing through the section of math books and I found this one book, which was all about one particular problem -- Fermat's Last Theorem.

Always try the problem that matters most to you.

I know it's a rare privilege, but if one can really tackle something in adult life that means that much to you, then it's more rewarding than anything I can imagine.