My personal experience is:
- Oracle is expensive and slow.
- MySQL is fast, though has had some teething issues over the years. It's gotten a little slower as they've added features, but is still pretty fast, and has gotten a lot more reliable. That said, change your default table type to InnoDB. MyISAM is a toy and should not be used in production. See MySQL Storage Engine Docs and Guide to MySQL storage engines.
- SQL Server had a decent edge on MySQL with regards to features at the beginning of the decade, but that has narrowed in recent years. I haven't used it much in production to really comment further.