New rails member on the block.

Excited to learn this new stuff while building my next app.
Question is, tutorials I looked at used sqlite, the default, which I think isn't a recommended choice for app development?

I'm looking at mongodb and even mysql (just because I'm familiar with it).
Mongodb doesn't make use of the activerecord (I don't know what its called, a class?). Is using activerecord so called best practice? http://www.mongodb.org/display/DOCS/Rails+3+-+Getting+Started

Is there a popular or recommended database that runs well with rails? Just like PHP is like always run with mysql?

Btw, I picked up my PHP skills here. Hopefully I could do the same with rails