If you're a .NET shop, and are either using a small database (i.e. Sql Server Express), or have the money for the full SQL Server, use it. SQL Server will perform better than PostgreSQL for most actions, and about the same as DB2.
PostgreSQL is fantastic if you need multi-platform support, are Linux-based, or need a free product that's not Microsoft.
I haven't used DB2 in over 10 years, other than running an in-house performance test vs. other databases (where it was about the same for a transactional database as Oracle/SQL Server, where were better than MySQL, PostgreSQL, etc).
