Some strange choices on here. We have some pretty hostile stations here in inner south London, Brixton and Elephant mainline stations spring to mind. The list would have been a lot longer in years gone by but Southern in particular has done a good job with previously rough stations like Tulse Hill and Peckham Rye through gating and more staff. Other side of the Thames, Essex Road has a personality all of its own....
Brixton certainly isn't as bad as it was, later in the evening even isn't as bad especially when there's a gig on at the Academy as strength in numbers and all that.
I'd say Plumstead is one of the worst I've been to in London - fare evaders galore and unsavoury types always hanging around the station, not to mention pulling passcoms when they get on trains.