Really Simple EPUB Reader is the simplest EPUB reader application imaginable.
It features some of the most sought-after features, including:
1) Opening EPUB files
2) Not crashing
3) Not looking terrible
4) Being really simple
It even features some extended "advanced" features like:
1) Having a browsable contents page
2) Showing images
3) Using nice fonts
Most simple books should be easily readable with this amazing reader. It is, most likely, the simplest EPUB reader in the known Universe.