Could we narrow the definition a bit? Obviously there are horrible shantytowns and areas of high criminality on every continent, but who wants to go there?
Instead, imagine that I am a jaded, eccentric millionaire who has already tasted all the conventional pleasures to be had in my home region, a dread of ennui and perhaps certain, ahem, peccadillos: where would I go?
Again, Phnom Penh. Although I suspect the Lagos nomination above is a good one even though I’ve never been there.
And probably Pattaya.
I’ll add Cartagena, Colombia to the list - that place is very exotic and parties a lot. I’m sure you can get anything there, but the most I ever asked for was rum and some very, very good coffee.
It is rebounding as a tourist spot. I was there in the mid-1990s, and it still had the air then of an outpost in a very lawless land.
So for the Top 7 we have:
Phnom Penh
Pattaya
Moscow
Tijuana
Amsterdam
Rio de Janeiro
[No clear consensus yet on number 7]
Common! There’s no way Amsterdam can be in the top 7 of the most sinful cities in the world?
There have to be some more suitable ones for the top ten.
I mean, people in Amsterdam are nice to you, travel on bicycles and the houses are way cool in their narrowness. And it’s pretty kid-friendly too.
Even Reykjavik is more sinful than Amsterdam… we have the strip-joints, heavy (by which I mean HEAVY) boozing and you see gang-fights* in the streets every couple of weeks.
*and by gangs I mean people ganged together fighting. Not gangs like the Crips, Bloods and so forth.
ETA: And I forgot that Icelanders are known for their promiscuity 
You’ve got to have Las Vegas on the list. Just because of the way it revels in its “Sin City” reputation. The latest ads (“What happens in Vegas…”) are basically advertising prostitution and encouraging people to come and cheat on their spouses. The gambling is almost an afterthought.
That’s one of my favorite Flashman books; I remember it quite well but now I’m gonna have to read it again. What do you think: Did Elspeth get it on with the pirate guy who’s name I’ve forgotten? How about with the French sailor(s)?
But of course she did! She must have. I’d be sorely disappointed otherwise. And everything he wrote about James Brooke, the White Rajah, was pretty much true. If I had to pick one person in history I could be, he’d be in the running if it weren’t that he probably really did get his balls shot off at an early age. In the link above, it says: “Throughout his life, Brooke’s principal emotional bonds were with adolescent boys, while he exhibited a total lack of interest in women.” I guess if you’ve had your package shot cleanly off, that would be true. But exploitwise, he was the original model for ripping yarns, and if he could have been a womanizer to boot, I’d probably choose him hands down.
But back to the topic at hand. I’ve never been to Amsterdam, but the wife spent a pleasant couple of months there while working on a project she was in charge of. I cannot reconcile her fondness for the city with sin status.
I’ll be honest with you. If Bangkok isn’t on the list, it shouldn’t count.
As Conan O’Brien put it, “Thailand is a country where you can have sex with a 14 year old prostitute…if you can convince her to come out of retirement.”
Funny. Sad. True. 
Ouyen.
http://www.walkabout.com.au/locations/VICOuyen.shtml
Absolute den of sin. And mallee roots.
(I’ll just go now, shall I?)
The strip clubs play a role in the traffic jam problem; for all I know, they cause the sinkholes, too.
And I don’t trust a strip club that’s owned by a gay man.
Never been to either place but Siam Sam says Pattaya beats out Bangkok.
Vladivostok . Remote, forsaken, cold. A port busy with Russian sailors and illegal Chinese and North Koreans desperate to make a rouble. All takes place in cyrillic, if that floats your boat.
Amsterdam
Amsterdam
Amsterdam
Las Vegas
Miami
Berlin
Not true. The police have done a lot in the past 10 or 15 years to adress child prostitution. If you do find a child prostitute in Thailand, Pattaya would be much more likely than Bangkok, but even then you’d have to search very hard. (Sadly, I will admit it’s largely only Western-oriented underage sex that’s been at issue. Woe be to the farang who dallies with a minor, the police will pounce immediately, but you rarely hear of Thai men being punished for it. It’s all an image thing, and Thai men having underage sex is not newsworthy internationally.)
Phnom Penh, on the other hand, is wide open, and they’ve never met a pervert they didn’t like. Except for this one idiot who somehow managed to get in wrong with the authorities and actually did get himself arrested for having sex with a minor a few years ago. I don’t know what he did exactly to piss them off, but he paid the judge a big bribe to assure his acquittal, and then the judge kept the money and sentenced him to 20 years anyway! The film of the courtroom session is a classic. The guy throws his chair across the courtroom after the verdict is read and is screaming, “But I paid you! I paid you! I paid you your money, you bastard!” as he’s beeing dragged away.
I think Singapore is long past it’s sinful days.
For what it’s worth, Heinlein later narrowed it down to the phrase “seven sinful ports” so I guess all the cities are on the water.
This list mentions Full Moon Parties, Koh Phangan Island, Thailand; Monaco; Ios, Greece; Gold Coast, Australia; Goa, India; Gatecrasher, England; Amsterdam, The Netherlands; Las Vegas, Nevada; New Orleans, Louisiana; and Pattaya, Thailand.
Is it really that easy? I mean, I’m not asking for a tutorial or anything, so don’t misunderstand me, but if one is in the mood for some underage lovin’ and is in Phnom Penh, does all he have to do is walk around and eventually he’ll find an underage brothel, while the police are looking the other way? Or is it a matter of having a local connection, having some money to bribe whatever cop might be looking your way, etc. etc.?
Montreal has a sinful reputation, but for… how can I put this? Fun sin. Safe sin, if you like, or at least safer sin. As in most of the more popular varieties of sin are widely available, but with little chance you’ll end up under a pier or something, and people are pretty chill about everything. So it’s not really on the list.
Well, I can’t say I have any personal experience with Cambodian brothels. But I have walked past some that had the girls spilling out the door, and some looked very young indeed, even considering that Southeast Asian ladies often look younger to begin with. And I do know that with Thailand’s crackdown on Western pedophiles, Cambodia has become THE place for them to go. Phnom Penh is still a Wild East sort of town. Although safer than a few years ago, walking the streets away from the main centers after dark, especially with a lot of money, is not recommended, to say the least.
The families in Cambodia are so poor that if they have a daughter to sell to a brothel, it’ll often be done, so the brothels do have a lot of underage girls mixed in with the ones of legal age; it’s probably not really a case of a brothel staffed entirely with underage girls (although that would still not surprise me). And then you have the Gary Glitter cases. He was popped for “sponsoring” a young girl from a family he’d befriended, and he would also have her bring her friends around. There are signs everywhere in Cambodia threatening dire legal consequences for indulging in underage sex, but the authorities there have yet to take a serious stand on it in practice. If you’re popped for it, it’s almost always a cop fishing for a bribe, or you’ve pissed off the authorities in some other way and this is how they can give you your payback.
And even I know the names of some of the more popular freelancer venues and that underage girls can be found hanging out there too, like Sharkie’s and Heart of Darkness, and I’ve never even set foot in those venues. Those places are legendary, for all sorts of reasons.
Amsterdam certainly doesn’t belong on the list (anymore).
Sure, you can buy weed or hashish in our coffeeshops but there isn’t that much in-your-face crime.
They are also cracking down on window-prostitution.
You might want to add Johannesburg, South Africa.