1001 Nights specializes in the surreal and will make you feel like you're tripping hard on acid during its short, but sweet 24 minute runtime. It featues three different animation styles: regular cell animation, sketch animation on paper, and these weird motion tween moments, and it all looks incredible.
Tenshi no Tamago features a lot of dark, gothic architecture and symbolism. The movie has little to no dialogue and is very slow but still manages to draw you into its world with its amazing character design and beautiful background art. Oh, and the music is really freaking good.
If you're an art maniac looking for something that's a bit more "out there" with its art direction, then both of these are a MUST watch! Oh, both are also made by the same character designer behind a lot of the Final Fantasy games: Yoshitaka Amano!