Protein of course (I'd be lost without it - whey concentrates, whey iso/hydrosylates, casein)
Fat burners - mostly just from caffeine but the non-stim thermos really help, it complements ephedrine nicely
X-Factor - although I eat red meat and fish, arachidonic acid is part of the muscle building cascade and should work for anyone
Creatine - monohydrate works great, magnesium creatine chelate and other salts work great for "non-responders"
Beta alanine - right up there with creatine monohydrate, has science to back it up and gives tons of muscular endurance
Pre workout supplements - call me a newb, but the nootropics, arginine and other "pump" ingredients may not build muscle but gives you a mental edge
I've spent way too much on supplements. I was desperate to get big and after spending thousands I realized that all you need is the staples. Depending on what sort of meat I'm consuming, I may or may not cycle a bit of creatine monohydrate, I always have protein shakes, sometimes take a fat burner with ephedrine or pre workout supp before the gym and take beta alanine. Just the basics are necessary, that and general health supplements like flax, fish oils, fiber, multivitamins and multiminerals, etc. Don't spend a fortune on other shit. Almost nothing works, and if it does work it's so underdosed that you have to double or triple the dosage to feel anything at all.