Forgive me if I sound cantankerous, but what the **** kind of question is that supposed to be? You're running a "dynamic web application", which does what? You've what kind of budget for hardware, licensing, people? Your admins are familiar with what environment? You need what kind of commercial support? You're expecting what kind of bottlenecks in your application? Are you a "consultant" looking for an easy answer without doing the research?

That being said, the web sites and forums of the projects you mentioned contain a lot of information regarding this topic, I suggest doing some research there. Without further information about your goals I'd lean a bit towards FreeBSD because it seems to be the preferred platform of the PostgreSQL and Apache people (I suspect largely for philosophical reasons). And I'd personally lean away from Solaris because I have zero confidence in Sun's future. But that's just me. Oh yeah, and in general I'd recommend looking at Debian.