- Microsoft SQL Server 2008
- Microsoft SQL Server 2005
- MySQL
We prefer Microsoft SQL Server 2008 and 2005 (if you don't want to upgrade yet) for MOST customers applications, however MySQL isn't a little toy that some claimed it once was - In fact it's QUITE capable for most sites, large or small - they have plenty of HUGE sites using it and if you think it doesn't scale it because you don't know the facts.
Why do we prefer Microsoft SQL Server? We've gotten better performance out of it after tuning indices professionally, however I do agree MySQL probably has as much potential in later versions for most web sites - If you are about to disagree and say Enterprise Edition is years ahead: Yes, but almost no customers run Enterprise Edition for their web sites.