What's an example famous artwork known for its simplicity?

Answer 1

 

Anything by Piet Mondrian would be a good example. 

 

You can also check these artists out:

- Mark Rothko 

- Kazimir Malevich

- Donald Judd

- Frank Stella

- Andy Warhol

Answer 2

You can't get "simpler" than Malevich, Suprematism, and his "Black Square," though the meaning of this work, according to Malevich and his confreres, is supposedly complex.