Hard Maple - has become the bat of choice for the majority of MLB players today. 
The use of bats with poor slope of grain has caused many dangerous breaks but the use of properly processed bats made from split Maple billets can drastically reduce the risk of multiple-piece fractures.
White Ash - For over a century, Ash has been one of the most popular species for baseball bats. Originally introduced by bat makers due to its ease of work-ability, Ash has stood the test of time based on its comparatively lighter weight, flex characteristics and availability.
Birch - has become much more popular among players at all levels of the game. Birch has proven to be a good alternative to Hard Maple and Ash with respect to performance. The supply of Yellow Birch logs suitable for baseball billets is more limited than Ash and Maple. The sap wood or live wood looks very similar to Maple and the heart wood has a brown color.