Serial Experiments Lain, Nekojiru-sou Recommendations
Both are very complex and may seem bizarre or too abstract to some. Both use unique and interesting ways of telling their story. But, Nekojiru-sou (Cat Soup) is only a 32 minute short, while Serial Experiments Lain is a 13 episode series.
Both are very surreal, bizarre, and abstract. While both are difficult to understand and open to interpretation, everything flows together making the madness enjoyable. Both have very interesting animation that should keep your attention.