Shadow Moses Island is Snake's home stage. It's a simple flat stage with a few platforms. However, the walls to the left and right are destroyable, which opens up walk-offs.
Sometimes Metal Gear REX or Metal Gear RAY show up in the background. They don't do anything though.
This stage's claim to fame, however, are the Snake codecs.
Super Smash Bros. Brawl- Snakes Codecs
The main reason why people chose this stage back in Brawl.