which is best storage engine for mysql?
My application is going to handle lots of users
And there is a lots of transaction on the database
And there will be a number of concurrent users

that would be InnoDB

You should look carefully at all of the engines available. InnoDB does offer transaction support, as does NDB, however, determining the right table is dependent on what the application is going to do. For example, session type information, such as data relating to login and session set data could be handled more efficiently with the memory engine.

I have a written a page summarizing the various engines and also the command on how to determine which engines are supported by your database server.click here