My top 3 movies are all very simple plot-wise
Eraserhead is about a man terrified by sex and parenthood but who finds himself responsible for a horribly deformed child
Mary Poppins is about a nanny who goes on magical adventures with children who have distant parents
2001 is about space exploration and a computer gone rogue causing problems and killing people
But all three films approach their simple plots so expertly, either through deep, complex emotion (Mary Poppins), stunning visuals and sound design (2001), or both of the above (Eraserhead)