I know that I'm probably opening a can of worms with this thread, but it's really just a simple poll to find out which database system you like best. I originally intended it to contain only MySQL and PostgreSQL, but in the interest of fairness, I added MS SQL into the mix as well. Vote away, and please be sure to explain your vote.
I'll be honest, I'm rather biased here as I've never used MS SQL and have only tried PgSQL a few times, and never seriously. But from what I've tried, PostgreSQL was slower, so MySQL gets my vote.
I use MySQL, it is simpler and easy to be managed. The reason I vote for MySQL is that any hosting provider offer Unlimited MySQL Databases whereas we have some restriction on the size or number of databases for MSSQL.
PostgreSQL has few more features than MySQL, still I would prefer MySQL only.
MySQL is always my choice. There is a lot of free scripts that work well with it. I remember back in the day when I had to do some nasty migration from MS SQL to MySQL... from there on its MySQL in my book.