Mine definately are the wheels. All of my cars have a ton of brake dust, and they are all hard to clean.
The easiest one.... takes about 10 min. for a wash (inside & out, using a bunch of tools)
Second to hardest... takes about 20 min. for a wash (inside & out, using a lotta tools)
The hardest one... takes about 30 min. for a wash (inside & out, have to climb under the car to clean)
And when it comes to polishing, claying, sealing it.... ahhhhh!