Best insecticide as in something you place on your skin to prevent bites and ticks? Nothing is better than pure DEET that I know of but you have to be so careful with that stuff as it will take the finish of most things it comes into contact with and can even damage some tent fabrics.

Edit: We began using OFF! Deep Woods Insect Repellent Wipes the last couple years and those things are so convenient and you don't have to worry about a bottle of that stuff breaking open in one your panniers. Seemed to work quite well including on the biting black flies of Wisconsin and would highly recommend. You can bet we'll be carrying some with us on the Minnesota Mesabi trail in a few weeks.