I voted stimpaks since that what I use the most as far as chems go. Followed closely by Radaway (which I didn't see an option for) since I seem to be a radiation magnet. On occasion I use Buffout if I need a strength boost for my loot and don't feel like dropping any of it.
Well they all have their uses my favorite combo is med x psycho and slasher that a whopping 50% increase in damage dealt and a 50% reduction in damage taken throw in a turbo and you've got what amounts to a win button, I don't use it much but every time I did it turned near certain defeat into a quick and bloody victory. I probably use stimpacks the most followed by the combat drugs like med x and psycho but I tend to save those until I'm in trouble and the stat boosters to pass the occasional skill check when I'm a few points off. I have to second hydra as a godsend on HC and med x gets even better since it can keep you going while your healing over time.
Stimpaks only, see no need for other drugs. Hydra is cool, but it seems sort of weird (i think it makes other people&creatures slow down, but not the environment)
I mostly use Slasher and Med-X in combat, sometimes Party Time Mentats for passing Intelligence and Charisma checks. Rad-X, Rad-Away and an occasional Stimpak go without saying.