What is the creature with the lowest neuron count that demonstrates cognition beyond reflexes?
Indeed, C. elegans nematodes (which are the ones you are talking about) do not show cognitive responses. AFAIK, Drosophila melanogaster is also able to learn and display some quite complex behaviors, but no cognitive functions. I believe the "simplest" organism known to display what could be called "cognitive" functions is the honeybee (see for example this article).

But as to your question:


  to demonstrate something more than mere scripted actions


I would also ask "what is a scripted action?" and to what extend a complex, cognitive response from a monkey or a human isn't also scripted (though through many different interacting pathways)?