Ultra Balls and Dusk Balls
These are your best bets right now. Unless 40 turns have passed.In which you will probably use a Timer Ball
Assuming it has 1HP left (False Swipe) with an Ultra Ball :
You have a 2.833% chance of capturing it per ball. Thus, you have at least a 50% chance of catching it within 25 balls and at least a 95% chance of catching it within 105 balls.
If you are , however, catching it in the Night
Dusk Balls will do better than Ultra Balls
>You have a 5.525% chance of capturing it per ball. Thus, you have at least a 50% chance of catching it within 13 balls and at least a 95% chance of catching it within 53 balls.
Or you can go for a Level Ball.
Since Ho-Oh is level 70 using a Pokemon quater or below 70 will increase Capture Rate by a bit.
>You have a 1.234% chance of capturing it per ball. Thus, you have at least a 50% chance of catching it within 56 balls and at least a 95% chance of catching it within 242 balls.
Or you can just use a Master Ball