1) Most Marketable (as in there are most jobs in this area)
2) Most Powerful (as in if I start a company, this language is scalable, efficient and reliable)

I know of the following setups:
1) LAMP (Linux, Apache, MySQL, PHP)
2) Ruby on Rails (I'm not too sure what this is, I just heard the name)
3) ASP.NET / IIS / Windows Server / SQL Server
4) Java Server Pages / Java Servlets with Tomcat Server and Linux

My guess is the most marketable is ASP.NET.

The most powerful is LAMP?