Can I use Not So Fast to counter Blitzcrank's ability?

Ruling

Yes, you can use Not So Fast to counter Blitzcrank’s triggered ability.

Not So Fast is designed to counter enemy spells or abilities that choose a friendly unit or gear. Because Blitzcrank’s "When you play me" ability chooses a unit you control, that unit is considered "friendly" from your perspective, making the ability a legal target for Not So Fast.

Reasoning

• Targeting: According to Rule 355.10, a game object mentioned in the text of an ability is a target. Blitzcrank’s ability ("...you may move an enemy unit to here") constitutes an effect that targets a unit you control.
• Interaction with Not So Fast: Not So Fast requires that the spell or ability be an enemy spell or ability that "chooses a friendly unit or friendly gear." Since an opponent's ability targeting your unit meets the definition of choosing a "friendly unit" from your perspective, it satisfies the criteria for Not So Fast.
• Rule Application: The ability does not fall under the exceptions in Rule 355.10.c, as the movement of the unit is the resolution of the effect, not a cost, trigger condition, or replacement effect.

Summary of Timing

If your opponent plays Blitzcrank:

1. Blitzcrank enters the battlefield.
2. His "When you play me" ability triggers and is placed on the chain.
3. The game state becomes CLOSED.
4. Because the ability targets your unit (a "friendly unit" from your perspective), you may play Not So Fast to counter the ability.

Note: I could not fully verify this ruling against the retrieved rules.