Maximum fraction of semen is contributed by:
Some important points regarding spermatogenesis:

Sperms are produced in: seminiferous tubules in testes.
Sertoli cells nurture the developing sperms.
Time for spermatogenesis: 64 days (Thymidine labelling records say it is 74 days).
Normal sperm count: 20 million per ml or 40 million per ejaculate.
Infertility is considered with a sperm count of less than 15 million sperms/ml.
Fructose is the main source of energy for sperms.
Prostaglandins suppress immune response against sperms in the female reproductive tract.
Zinc serves to help to stabilize the DNA-containing chromatin in the sperm cells.
Acrosomal cap houses the various enzymes needed to penetrate the zona pellucida of the oocyte.
Seminal Vesicles
Prostate
Bulbourethral glands
All contribute equally