What is the most effective exterior insecticide?
I have tried many different compounds, and found one to be particularly effective with low human toxicity:

Bifenthrin
    - A pyrethrin, only slightly toxic to mammals, nontoxic to plants

Talstar Pro -- Liquid concentrate, mix your own and spray

Ortho Home Defence Max Granules -- Granules, apply by shaking along perimeter

I use the liquid concentrate because it is much more cost effective, but the granules are much easier to apply.  I have kids and I chose this compound due to it's relatively low mammal toxicity.  I'm unsure what else is in the granules besides the active ingredient, and that also deters me from using it.

You can get the granules at your local home improvement store, and they also sell it in liquid form in a self-contained spray bottle but unless you fancy pulling a spray bottle trigger 1000 times to apply it, I would skip that option altogether.