Go to the Gare Cornavin. And Parc de Contamines is meant to be 'the place' to score drugs. Another place near the Russian Orthodox church in Tranchees is a prostitute area (for want of better description).

By the way, it is illegal to photograph people without their permission or knowledge in Switzerland.

I believe consent is required only if you take photographs of one or more identifiable persons. It's hard to catch a decisive moment or a candid exchange in a public place if you have to first explain to people who you are and what you're doing... For other kinds of pictures (e.g., environmental portraits or even "traditional" ones), I always ask. It often helps humanize the subject, especially in this context.

Hello! Thank you for replying so promptly. The Cornavin area can be shady, indeed... Prostitutes are not a good focus because I get harassed by the pimps -- and they're not friendly at all, especially when the girls are minors... I'll check out Parc des Contamines -- thanks!

Paquis has an interesting fauna of drug dealers, pimps, and hookers, too.

If you're more into gypsies, beggars, and pickpockets, why not take a stroll in Plaine de Plainpalais.

The area around the tram stop Stand/Paladium looks dodgy too