Python often leaks memory, the GC being built on top of reference counting to solve cycles, isn't compacting and can also be left disabled for performance, which is a trap. This is made worse by native libraries that also leak. It has gotten better, Python apps being one of the most deployed. But I haven't seen a Python app that's memory efficient yet, except for short lived shell scripts.
PHP uses tens of MB per request, which due to its "shared nothing" policy means you need several GBs of RAM for serving a Wordpress website that can withstand traffic spikes.
Many companies actually deploy their Ruby apps on top of the JVM, via JRuby, due to better memory usage.
> the biggest buck for the bang actual is dotnet
Not in my experience, but granted I haven't tested .NET Core yet.