This depends on what you mean by "most common."
The most commonly transmitted STD is HPV. According to the British Journal of Obstetrics and Gynecology, 46 percent of teenage girls acquire HPV from their first sexual relationship. Such high rates of infection are widely reported, and scientists estimate that over 50 percent of sexually active men and women have been infected with one or more types of genital HPV. Young women are most at risk of being infected. For example, 40 percent of sexually active girls between the ages of fourteen and nineteen are currently infected with HPV. The numbers are even higher for women aged twenty to twenty-four (49 percent)! Among all women this age bracket has the highest rate of HPV. The prevalence of HPV also varies according to marital status. For example, only seventeen percent of married women are currently infected. However, nearly half of all women who are living with their boyfriends are infected with the virus. Such high numbers seem almost unbelievable. But one must remember that most people with HPV will not show symptoms or suffer as a result of it. Although HPV is incurable, this does not mean that it is permanent. In fact, HPV will usually go away within two years. So despite the fact that most women have been infected with HPV, only 27 percent currently test positive for the virus.
The most common STD in terms of current infections (prevalence) is herpes (HSV). About one in six people are infected with genital herpes type-2. Among sexually active singles, the percentage of those infected with herpes is even higher: between 30 and 40 percent! While these numbers may seem astoundingly high, one must realize that nine out of ten people who have genital herpes are unaware that they are infected. Despite the fact that more people contract HPV than Herpes, Herpes is more common because it is incurable.
The most common reportable STD is chalmydia, because doctors don't have to report diseases such as HPV, Herpes, and many others. In 2005 nearly a million cases were reported, but most cases go undiagnosed, and so the Centers for Disease Control estimate that about three million infections occur each year in the United States..
. S. Collins, et al., "High Incidence of Cervical Human Papillomavirus Infection in Women During Their First Sexual Relationship," BJOG: An International Journal of Obstetrics and Gynaecology 109:1 (January 2002): 96-98.
. L.E. Manhart and L.A. Koutsky, "Do Condoms Prevent Genital HPV Infection, External Genital Warts, or Cervical Neoplasia?: A Meta-Analysis," Sexually Transmitted Diseases 29:11 (November 2002): 725-735; Division of STD Prevention "Prevention of Genital HPV Infection and Sequelae: Report of an External Consultants' Meeting," DHHS, (CDC), 7.
. Eileen F. Dunne, et al., "Prevalence of HPV Infection Among Females in the United States," The Journal of the American Medical Association 297:8 (28 February 2007): 816.
. Dunne, et al., 813-819.
. A.B. Moscicki, et al., "The Natural History of Human Papillomavirus Infection as Measured by Repeated DNA Testing in Adolescent and Young Women," The Journal of Pediatrics 132:2 (February 1998): 277-284; E.L. Franco, et al., "Epidemiology of Acquisition and Clearance of Cervical Human Papillomavirus Infection in Women from a High-Risk Area for Cervical Cancer," The Journal of Infectious Diseases 180:5 (November 1999): 1415-1423.
. L. Koutsky, "Epidemiology of Genital Human Papillomavirus Infection," The American Journal of Medicine 102:5A (5 May 1997): 38, as cited in Centers for Disease Control, "Tracking the Hidden Epidemics, Trends in STDs in the United States 2000," (6 April 2001), 18.
. Dunne, et al., 815.
. Fujie Xu, et al., "Trends in Herpes Simplex Virus Type 1 and Type 2 Seroprevalence in the United States," The Journal of the American Medical Association 296:8 (August 2006): 964-973.
. Joe McIlhaney, M.D., Safe Sex (Grand Rapids, Michigan: Baker House Books, 1992), 100.
. D.T. Fleming, et al., "Herpes Simplex Virus Type 2 in the United States, 1976 to 1994," New England Journal of Medicine 337 (16 October 1997): 1105-1111; P. Leone, "Type-specific Serologic Testing for Herpes Simplex Virus-2," Current Infectious Disease Reports 5:2 (April 2003):159-165.
. Division of STD Prevention, "Trends in Reportable Sexually Transmitted Diseases in the United States, 2005," (CDC), 1.