When a Deflect says 'opponent must pay to choose me with a spell', but my spell hits all enemies, do I still have to pay?

No, you do not have to pay the Deflect cost in that situation.

According to the FAQ in the rules:

"Deflect only needs to be paid when you directly target or choose a unit with Deflect. If a spell targets a battlefield or affects all units without choosing specific units, you do not pay Deflect."

Key Nuances:

• Targeting vs. Affecting: Deflect is a mandatory additional cost that only triggers when a spell or ability specifically "chooses" or "targets" the unit with Deflect.
• Area Effects: If a spell says "deal damage to all units" or "all enemies," it is not choosing individual units, so Deflect does not apply.
• Exceptions: If a spell allows you to split or distribute damage, and you choose to assign any amount of that damage to a unit with Deflect, you must pay the Deflect cost for that unit because you have made the choice to target it.

As stated in Rule 735.1.c, Deflect is functionally short for: "Spells and abilities an opponent controls that choose me cost an amount of Power equal to [Deflect Value] more to play as an additional cost for each time they choose me." If your spell does not require you to choose the unit, the cost is not incurred.