(I think that for a question like this with the answers being entirely personal, the voting is of little or no significance.)
For me there are so many that I hardly know where to begin. Initially, Martin Gardner. Among those I knew personally: my undergrad profs (espcially I.M. Singer) who taught me what math is. Then Bill Thurston, with whom I shared an office in grad school. Stephen Smale, my de facto co-thesis advisor.
Notably Gauss, Riemann, Klein, Poincaré, Milnor.
Above all, my thesis advisor, Morris Hirsch, with whom I've had a continuing connection since 1970.
