Every pitcher has his "out" pitch. For some its a fastball and for others its a hard curveball. For Lincecum its his change-up. It is by far his best pitch in my opinion. Every pitcher wants an edge- whether its velocity or movement or just plain deception. Lincecum has a little of all which is what makes him great.
But I like to think of the whole fear factor in batters- let them state what they fear in a pitcher. For batters facing Lincecum, it is obvious that they fear his CU- they even state that and how difficult it is too pick up. If a pitchers best pitch is what is defined as instilling fear in a batter then the CU is definately Lincecums best pitch he has.