They come a lot weirder than "Withnail and I." (Coincidentally, I just saw that movie Friday night.) Here are some very weird ones:

Any of Federico Fellini's later movies, like Satyricon (1969).

Caligula (inexplicably listed at IMDb as Caligola), 1979, arguably the worst movie ever made. Certainly the worst movie ever made by such well-known, good actors (Malcolm McDowell, Helen Mirren, Peter O'Toole, John Gielgud).

Eating Raoul (1982). I can endorse this one. It's funny. Intentionally.

Anything by John Waters, especially his early movies like Pink Flamingos (1972) and Female Trouble (1974).

O Lucky Man! (1973) – another Malcolm McDowell movie.

Gummo (1997). Frankly, I loved this movie, but I'm reluctant to recommend it because I know a lot of people will hate it. My wife, for example, left after about 10 minutes, because of the graphic (simulated) cruelty to cats.