To be honest, I think you summed up The Fifth Element’s design appeal much better than I could. The streams of traffic and the buildings felt “real”; claustrophobic, functional and without frills, and they fitted my image of a future New York perfectly. I don’t know how much was owed to Blade Runner, but it steered away from being too distopian through the sheer colour and spots of tongue-in-cheek humour in the film.
City of Lost Children was altogether more downbeat. A nightmarish world of disconcerting colours, a lack of light and (again) a claustrophobic mess of buildings, yet without people. I liked the retro feel, which generally managed to steer away from items that might have associated the film with a particular “real world” place or time. I think both films were also helped by excellent costume design - was it Gaultier behind both?
I would add that I really like the set design in the second Batman film; I still enjoy it more than the first (not to stray too far off-topic, but Keaton was my favourite Batman, and Walken and Devito were a more interesting combination than Nicholson was on his own in the first film). Maybe it’s a bit of an obvious Tim Burton trademark by now, but I liked his usual semi-gothic style.