In addition to the mainline figs I have air warrior x3, rainmakers, sunstorm, PE frenzy/rumble, mp3 soundwave, FP Bruticus add-ons, custom pounce/wingspan, custom insecticons, custom Menasor, and custom Devs. Pretty decent army 

 

I army build more cons than Autobots. Its that simple.

Oddball Soldier molds and decos that can work as their own unit rather than a individual.

Multiples of Stormcloud, Scourge, Terradive, or Hailstorm.

Its easier to do when you find things on sale. Animated Oil Slick and Sunstorm for example I found on clearance a lot so they buff up the decepticon ranks well.

It fits the logic that parents just don't buy their kids that many decepticons. They always have clearance tags. Optimus Prime and Bumblebee are the first to leave the pegs.