An enemy uses Smite in my Ruined Rex. If I want to use Baited Hook in the Rex, what is going to happen?

To determine what happens when you attempt to use Baited Hook on a Ruined Rex that has been targeted by an enemy Smite, we must look at how replacement effects and the "kill" game action interact.

The Ruling

If the enemy uses Smite on your Ruined Rex and you activate Baited Hook in response targeting the same Ruined Rex, Baited Hook will fail to play a unit from your deck.

Why this happens

1. Smite's Replacement Effect: Smite contains the effect: "If it would die this turn, banish it instead." This is a Replacement Effect. When the effect that would cause the unit to die occurs (such as your Baited Hook's "Kill a friendly unit" instruction), Smite’s replacement effect intercedes.
2. No "Death" Occurs: Because the replacement effect changes the event from "dying" to "being banished," the Ruined Rex never actually completes the "die" game action (moving from the board to the trash).
3. Baited Hook Requirement: Baited Hook specifically requires you to "Kill a friendly unit." Rule 359.3.e.12 and several RiftJudge FAQs (e.g., FAQ #4178, #8203) confirm that if a save effect (or in this case, a replacement effect that changes the zone) prevents the "kill" action, there is no "killed unit" to reference.
4. Null Might: Since no unit was actually killed, any reference to the killed unit's Might value returns null (Rule 359.3.e.12). Consequently, you cannot satisfy the condition of finding a unit with "Might up to 1 more than the killed unit."

Sequence of Resolution:

1. Activation: You activate Baited Hook targeting your Ruined Rex.
2. Resolution Begins: Baited Hook attempts to perform the "Kill a friendly unit" instruction.
3. Replacement: Smite's replacement effect triggers, changing the "Kill" action into a "Banish" action. The Ruined Rex is banished.
4. Check: Baited Hook checks for the "killed unit" to determine its Might value to finish its instruction. Because the unit was not killed, the check returns null.
5. Completion: You will still look at the top 5 cards of your main deck (as you do as much as you can), but you will be unable to play any unit because there is no valid Might value to compare against. You then recycle those 5 cards.

Note: Because the unit was banished and did not die, your Ruined Rex's Deathknell ability will not trigger.

This is my interpretation based on the mechanics of replacement effects (Rule 370) and consistent rulings for Baited Hook (RiftJudge FAQs #4178, #8203, #11249).