If you were looking for a de facto direct replacement to MS SQL Server and just didn't want to have to "think" much about anything, the obvious choice is PostgreSQL. It is one of the world's most powerful, mature and respected database platforms. It is insanely fast, scales insanely big, is supported on the most enterprise style deployments, does relational the same way that MS SQL Server does and so forth. It is the most direct competitor. And you have loads of top end support options for it. So if you want a safe "starting point", start there. It's hard to go really wrong. But best to do an evaluation of needs as there are just so many great options
