i always have three on hand.
bsn no-xplode because the shit just works. first time i tried it i expected nothing, but damned if i didn't have more energy at the end of the workout than i ever do. stronger? not particularly, but the pumps are good and it definitely makes you more alert.
bsn cell-mass, just because creatine does work and the GNC around here has retarded sales like 35% off then buy one get one 50% off on top of that.
some kind of protein, mostly because if you add up the calories you'll never have a cheaper source than protein powder. i got a tub of weight gainer, 730 cals a pop, 18 servings, for about $25. there's no other area where you can get 18 730 calorie meals for under $30.