Paul is right. How can you forget someone who ran in multiple classes and still won. My vote goes to Ago, a real superman (though Rossi is a genius on two wheels).
Career summary
Year Team 350cc 500cc World titles TT wins
1963 Moto-Morini - - - -
1964 Moto-Morini - - - -
1965 MV 3 wins 1 win - -
1966 MV 3 wins 3 wins 500 cc 350 cc
1967 MV 1 win 5 wins 500 cc 350 & 500 cc
1968 MV 7 wins 10 wins 350 & 500 cc 350 & 500 cc
1969 MV 8 wins 10 wins 350 & 500 cc 350 & 500 cc
1970 MV 9 wins 10 wins 350 & 500 cc 350 & 500 cc
1971 MV 6 wins 8 wins 350 & 500 cc 500 cc
1972 MV 6 wins 11 wins 350 & 500 cc 350 & 500 cc
1973 MV 4 wins 3 wins 350 cc -
1974 Yamaha 5 wins 2 wins 350 cc -
1975 Yamaha 1 win 4 wins 500 cc -
1976 MV 1 win 1 win - -
1977 Suzuki - - - -
Totals 54 wins 68 wins 15 Championships 12 TT wins