In March 2011, I create a poll in Zabbix Forum with this question:
It received 92 votes and MySQL was the most voted with 71.74% followed by PostgreSQL with 21.74%
Personally I found many problems with Oracle 11g when reach more than 1000 nvps (new values per second).
My results with MySQL was very good reaching more than 3500 nvps.
Unfortunately I still could not test PostgreSQL, but there are many positive posts in Forum.
And you, which database are you using?