As everyone here keeps saying, "there is no such thing as best". It depends entirely on what you're going to be using. That being said, I probably would recommend Oracle or SQL Server 2000, since those seem to be rather prevelant. DB2 (IBM) has a good series of exams as well.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!