After Working at GNC for many years, The two most popular that we would sell are: C4 and Vault
My personal favorite until my store stopped carrying it and/or was taken off the market was Black Powder.
Any more, I have gotten away from caffeine completely and have been taking GNC Preworkout Amino Complex. Other than that, Some simple Carbs with a high glycemic Index and some sore of Free form Arginine.
http://www.gnc.com/pwr/product-revie...-Servings.html