Can you tell me what is the best exercise for a mesomorph?

Depends on goals: For sports fitness, play the game and work the muscles. Aerobic fitness has little to do with body type, and gives endurance and speed over mass. A mesomorph may be defined as big-framed and/or an easy-gainer of mass -- these are contradictory and confusing. Resistance training as for anyone else. Especially, eat plenty of protein and otherwise eat moderately. Best wishes.