Best is defined as using parts others discard for better performing parts because they come cheap?
I watched the best Pro Touring and highly modified cars at Road America last weekend autocross. In the mix were a Lotus Elise, monster build Nissan GTR, highly modified Z06 Vette and more. The drivers are exceptionally skilled. You cannot find a competitive stock vehicle at the OPTIMA because they can't compete. Not a stock ZR1 or a Porsche GT3 or a Ferrari or Lamborghini have made the Top Ten at the Invitational. Pedders USA 5th Gens have never not been in the Top Ten.
Can you upgrade a base 5th Gen with off the shelf GM parts, of course. Does that get you the best bang for the buck? Not in my book. If the parts you bought are not the best of the best, you paid too much.