ADO.NET Entity Framework in asp.net suited for different database.
Performance in Enterprise level web application or medium level web application.
Is it the best alternative to other orm tools like linq, NHibernate.
Please see this other question which has an answer that links to a bunch of other questions on this topic:
NHibernate, Entity Framework, active records or linq2sql
It's difficult to answer this specific question more specifically given how the question is written.