I rank them thusly:
Oracle
PostgreSQL
DB2
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MS SQL
Most of the testing done by the folks on the pgsql mailing lists has shown that on similar hardware, PostgreSQL comes within 10 to 20% of Oracle's performance under heavy parallel load, beats it in a few areas, and loses in a few others.
Really, other than not having a built-in replication or PITR system, pgsql is a match for Oracle or DB2 for most applications.
MS SQL server is still not capable of scaling as well under heavy parallel load as either of the other three. I.e. it has a tendency to implode under heavy parallel access in such a way as to make me most uncomfortable using it for critical business applications.