Riemann's paper, "On the number of primes less than a given magnitude," is the reason why I decided  to study mathematics (at the graduate level and beyond).  I read the paper as an undergraduate and I was very impressed by the techniques that Riemann used to study the properties of the prime counting function.  In particular, I was blown away by Riemann's use of complex analysis, Fourier analysis, and asymptotic analysis to study a problem in number theory, which I thought was a distant area of mathematics.  This paper is truly a work of art and is less than 10 pages.