I use Baited Hook to kill a unit, does the unit's Deathknell ability trigger first or can I choose to do the Baited Hook 5 card reveal to play a unit first?

According to RiftJudge FAQ #8567, when you use Baited Hook to kill a unit with Deathknell, the resolution order is fixed by the rules of the chain (LIFO - Last In, First Out). You do not have a choice to resolve the reveal effect before the Deathknell trigger.

Sequence of Resolution

1. Activation: You activate Baited Hook by paying its cost and targeting a friendly unit.
2. Resolution Begins: Baited Hook's effect begins to resolve. It kills the target unit.
3. Deathknell Triggered: Because the unit has Deathknell, its trigger becomes a "pending item" on the chain. Per Rule 428.1.a.1.b and Rule 808.1.d.2, the Deathknell trigger is added to the chain before the Kill instruction is completed.
4. Baited Hook Continues: Baited Hook continues resolving its remaining instructions (looking at the top 5 cards and playing a new unit).
5. New Unit Finalizes: Any unit played via Baited Hook enters the board and finalizes immediately. If that new unit has a "When you play me" (WYPM) ability, it is added to the chain as a pending item.
6. Resolution (LIFO): The chain resolves in reverse order:
   • Any WYPM ability from the new unit resolves first.
   • The Deathknell trigger resolves last.

In summary: You do not choose the order. The Deathknell trigger is created first, but because the chain resolves in LIFO order, the Deathknell trigger will always resolve after the effects of the Baited Hook have finished.