How to choose a database

I understand that people in this forum have rich database experience in one of the best rdbms in the world i.e. Oracle. And thus wanted to ask, have you been into situations in which somebody in your organization asks to evaluate or choose another of the cloud native databases(may be OLTP or OLAP) apart from Oracle. Say for example I have seen situations in which companies are moving existing applications or building new applications on to the AWS cloud so they don't want to be bound to Oracle rather trying to utilize some databases which is native to that cloud(say RDS , Aurora, redshift) or may third party databases which are available in multiple clouds(Yugabyte, Cockroach, Snowflake etc ). And we know Oracle is not available in Aurora and RDS has some max storage limitation for oracle and additionally Oracle exadata is not available in AWS too.

So my question is,
1)Considering databases are something going to stay long and not frequently changeable, how do you evaluate/choose the best goto databases for OLTP(for 100's terabyte scale) and OLAP(for petabytes scale) applications for your organizations(say financial industry where ACID matters) at the current time when there are many databases in the market and also claiming to be the best in the field?
2)Many things come to mind, like we should see/test the isolation level, ACID and CAP etc. But is there a quick guide/workflow available which provides these accurately or we need to test these all by installing the database one by one manually?

Regards
Pap

--
http://www.freelists.org/webpage/oracle-l