How does Teemo's defense ability work on a chain, do I have to choose which unit to target first, or do I just reveal my 5? Where is the window of that chain to react/counter to?

Yes, you choose the target first, then the opponent can react, then you reveal. Here's the exact order:

1. Teemo gains Defender designation (during combat setup, or immediately if revealed from Hidden mid-combat per FAQ #8317).
2. The "When I defend" trigger goes on the Chain as a pending item.
3. During finalization (while the trigger is still pending), you choose the target — "an enemy unit here." Targets are locked at finalization, not resolution (Rule 352.4.b).
4. The trigger becomes a Chain Item. Now the opponent gets priority — since they control the newest item on the Chain (Rule 340.4), they can play Reaction-speed spells/abilities. This window opens before any cards are revealed.
5. If both players pass, the ability resolves — you reveal 5 cards, deal 1 per Hidden card revealed, then recycle.

Key takeaways:

• You cannot wait to see what you reveal to then pick a target — the target is chosen in step 3, before you ever see the cards.
• The opponent cannot wait until after you reveal to react — their window is in step 4, before resolution.
• If they kill or recall Teemo with a Reaction in step 4, the ability still resolves since it's already on the Chain (it just won't deal damage if the target is gone or Teemo's zone changed prevented the reveal).
• And no, they can't play Action-speed spells during this window — only Reactions are allowed in a Closed State.