SubSonic is a fine ORM, but it lacks many features that Entity Framework 4 and NHibernate have. SubSonic also isn't moving forward as quickly as EF and NH.
It is possible that SubSonic will be the best .NET ORM two years from now, but that seems highly unlikely at present.
There is no one "right" or "correct" answer when choosing a .NET ORM. NHibernate seems like the safest choice right now, while Entity Framework 4 is a good fit for companies that prefer solutions from Microsoft. SubSonic 2 & 3 will work just fine in many situations and it may be better for your project to just maintain what is already working.