UPDATE 2016
Six years later things are VERY different. NHibernate is all but abandoned, other alternatives were abandoned (eg Subsonic), Entity Framework is perhaps the most common full-featured ORM, and people have been moving to micro ORMs like Dapper for years, to map from queries to objects with a minimum of overhead.
The application scenarios have also changed. Instead of loading and caching one big object graph at the expense of memory and performance, Web Services and REST APIs need to service a high number of smaller requests. This means that a full ORM's overhead is no longer acceptable.
This means that patterns and techniques like Active Record, transaction per request etc have become throughput and scalability killing anti-patterns
One of the most important features nowadays is asynchronous execution , to reduce thread and CPU waste due to waits. NHibernate never made this transition.
Original Answer
Define "best": Is it the most mature, the one with more documentation, bigger community, more mainstream?
NHibernate is more mature, feature rich, with a more advanced community and not likely to be discontinued when MS decides to break compatibility again. Entity Framework is more mainstream and is supported out-of-the-box. You will find more beginner books for EF, more advanced books for NH.
A good option would be to try one of the simpler ORMs like Subsonic and move to more advanced ORMs once you understand how ORMs work, what are the various pitfalls, what SELECT N+1 means [:P]
Just don't try to create your own ORM, there are several dozens out there already! Subsonic, Castle ActiveRecord, NH, EF (of course), LLBLGenPro...
