Sleaziest would be Pattaya, I think all will agree on that.
Highest rate of HIV / STD, I think some actual number and stats would be fair if anyone post any such conclusion.
Back in 2008 there was an article posted in Phuket Gazette that Phuket (that year) had the second highest rate if HIV infections in Thailand, behind only Ranong.
I don't know if any of that has changed over the past 3 years.
The page is no longer available, but here is the article:
Phuket second highest HIV infection rate in Thailand
PHUKET CITY: -- Phuket now ranks second among Thailandโs 76 provinces in terms of HIV infection, with the large number of nightlife entertainment venues and the sex industry seen as the cause.
Dr Wiwat Sitomnoch of the Phuket Public Health Office reported that the incidence of HIV infection among teenagers and young adults in Phuket is rising, as are the number of cases of gonorrhea and other sexually transmitted diseases. About 90% of cases are the result of unsafe sex practices, he said.
The Public Health Ministryโs Department of Disease Control has been keeping statistics of HIV/Aids cases in public and private hospitals in Thailand since 1984.
As of August 31 this year, 335,414 cases had been reported, with 91,839 deaths.
In Phuket, the number of cases reported from public and private hospitals between 1989 and October 2008 is 5,864, with 1,531 deaths. The number of cases among men is greater than among women by a factor of 2:1.
In a breakdown of the Phuket statistics by age, the highest incidence was among people aged 25-39 years. In this range, the highest incidence was among 30-34 year olds, with an incidence of 25.67%, followed by 25-29 year olds (22.10%) and 35-39 year olds (18.02%).
By profession, the most cases were among people who describe themselves as โworkers for hireโ, followed by domestic workers, the unemployed, retail vendors, company workers, manual laborers and 'fisherfolk', respectively.
Dr Wiwat said Phuket has the second highest infection rate in Thailand, second only to Ranong, for the period from 2007 to August 31 this year.
-- Phuket Gazette 2008-11-20