Ask HN: Which is the best PHP & MySQL CMS out there?

Which is the best cms out there, based on PHP & MySQL?

I am looking for:

1) relative stability ( can't afford to experiment for this part now)

2) very good online/published support

3) ease of starting (for a guy new to CMS but is comfortable with using PHP/MySQL, though not an expert)

4) capable

5) flexible (in terms of how easily the standard templates can be hacked, modified, etc)