Can I counter Hostile Takeover with Not So Fast?

Yes, you can counter Hostile Takeover with Not So Fast.

According to the official FAQ for Hostile Takeover:

"Hostile Takeover does choose a target... You must pay Deflect costs to play Hostile Takeover on an enemy unit with Deflect."

Because Hostile Takeover is a spell that chooses (targets) a unit, it meets the requirement for Not So Fast, which states:

"Counter an enemy spell or ability that chooses a friendly unit or gear."

Key Details:

• Timing: You must play Not So Fast while Hostile Takeover is still on the chain (before it resolves).
• Effect: If countered, Hostile Takeover does nothing and is placed in the Trash. The energy spent to play it is not refunded (Rule 412.1).