If I could only choose one - poison tail (it can do it all... though some others are more specialized);
If two - arky and casting (better choices for specifically flipping and swimming);
If three - arky, casting, and shake-it;
Four - three above and poison tail (now used primarily as wacky jig)
Five - four above and weighted hook (for jerk baits and swim baits)
Six - five above and another plastic mold