Which embedded database has maximum SQL compliance, and concurrency support?

One happy firebird user looking for a cool Sql Embeeded Database and is ditching Microsoft Access too

I did trials with Firebird, and it really does seem very good, zero admin, and SQL compliant. It is fast, and I had no problems in any of the typical queries I could try on it. I am very satisfied with it, and hopefully will use it for the project for which I raised this question.Hopefully Advantage Server too will be good, but could not get time to review it. After reviewing/using Firebird do not feel any need to try anything else.