Really I'd find the best one for whatever language you're strongest with.
My absolute favorite, bar none, is Ruby's DataMapper.
In PHP, the best ORM project is certainly Doctrine. Doctrine can be a little overwhelming at first, but it's worth it. However if you found yourself just not pursuing it because of its complexity, I've found that the Kohana framework has a quaint, easy to use ORM. Not a bad start.
LINQ is not an ORM. There is LINQ to Objects and LINQ to XML, which query collections and XML documents respectively.
The first ORM over Linq from Microsoft was Linq to SQL. This is now deprecated, and the second one is called Entity Framework (EF), which was a more ambitious but slower-moving project. In the latest release, EF 4.1, it's actually quite good.
NHibernate is still very popular, in part because over the confusion over Linq to SQL and EF, in part becuase it was there first and is very mature.
There are a whole lot of other, less-well-known .Net ORMS - SubSonic, SimpleData ... just google ".net orm". But for new work, most people would only consider EF and NHibernate.
.NET also has NHibernate. I'm not intimately familiar with the pros/cons of LINQ to SQL vs NHibernate, but I believe it gives you a lot more control over things like pre-loading associations, etc (while requiring a lot more configuration).
One con of LINQ to SQL is that it only supports MS's DBs. LINQ to Entities is the more flexible version. Though you have to go whole hog and use Entity Framework. There is also a active record implementation for .NET called castle active record.
> One con of LINQ to SQL is that it only supports MS's DBs.
This is not true for Entity Framework (which is essentially the newest iteration of Linq2Sql.) I have successfully used EF with SQLite in production, and it appears to have a MySQL driver as well (I honestly don't know how well the MySQL driver works.)
That is what I meant by LINQ to Entities, which is what I thought they were calling the new version. There are plenty of DB drivers(I think they call them providers) for EF.
I've been using datamapper recently - I like it, but would be interested to hear what makes you like it so much over the others (which I haven't used, except for a smattering of ActiveRecord).
