The best ORM for C#

Interesting, but what about ORM for .NET I can definitely say that the best (really the best), than were given to use is LightSpeed from MindScape. In the first place because when designing I follow the paradigm of Domain-Driven Design, and there it is supported on 5+, and also thanks amazing speed.
But it has one global drawback — it is not free (Express version of I is not taken seriously, a limit of 8 objects the model is too constrained).
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Further — anyone who approaches this issue, is bound to face a fork — Entity Framework or NHibernate. Here is a matter of taste, personal preferences, and approach to development. NHibernate is much more Mature and "ready" than EF, but EF has visual designers and other MS-ovskie Goodies. NHibernate, as mentioned above, it is better to use Fluent-NHibernate, if you don't like to write tons of XML :)
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In addition, if you need to use the ActiveRecord pattern (it may be applicable in your case) — have to look at NHibernate and SubSonic.

above recommend NHibernate, but I believe that the author, who is not familiar with ORM, NHibrenate excess. The authors recommend to use Entity Framework 4, which is your one-stop visual editor and full support for LINQ (which is lame NHibernate).

Used NHibernate, Entity Framwork, Linq2Sql, Gentle.Net. IMHO NHibernate is currently the most flexible and the most advanced ORM. Immediately recommend to use it in conjunction with FluentNhibernate. ENtityFramework gradually is selected according to the capabilities of NHibernate, so if there is a desire to use anything directly from MS, use it.

NHibernate.
Since I use only them, the other I can not advise... you can Still try EntityFramework, but I do it after using NHibernate, did not like the display configuration of the objects in the database.

Perhaps it's all the same from a task depends heavily on the answer. For small projects it is suggested to BLToolkit. So I recommend the website ormbattle.net