In both Smogon Singles and Monotype singles, Lycanroc-Dusk is the best Lycanroc form.
Singles
I know you said OU, but none of the Lycanroc forms are viable in OU. They are outclassed by all of the physical wallbreakers listed on the OU Role Compendium
Since the Lycanroc Forms are in different tiers, I'll list the tiers they are in, why they are there, and what they do in said tier.
Lycanroc-Dusk
Lycanroc-Dusk is in the tier UU. The reason it is higher than the other Lycanroc forms is it's access to Tough Claws. Tough Claws boosts its priority move Accelerock and coverage moves, which makes Lycanroc-Dusk a good revenge killer and all-out attacker in UU. However, it has poor bulk and a poor defensive typing, meaning it requires support to bring it in. Despite that, Lycanroc-Dusk makes for a good offensive Pokemon in UU.
Source for what Lycanroc-Dusk does
Lycanroc
Lycanroc is in the tier PU. Its only useful ability is Sand Rush, which can be utilized on Sand teams in lower tiers, but they aren't really used. Outside of Sand, Lycanroc still has a good speed stat along with a good attack stat, and access to Accelerock, which lets Lycanroc function as an all-out attacker and revenge killer, just like Lycanroc-Dusk. The thing that holds Lycanroc back is the same problem as its Dusk Form, as well as struggling to get past physically defensive walls.
Source for what Lycanroc does
Lycanroc-Midnight
Lycanroc-Midnight is (PU), which means Untiered, which is also known as ZU. Lycanroc-Midnight is used as a Choice Band user as well as a Swords Dance wallbreaker, which is used to clean late-game. Lycanroc-Midnight has access to No Guard Stone Edge as well as a priority move in Sucker Punch. The things that make Lycanroc-Midnight the worse of the Lycanroc forms competitively is due to its lower speed stat, which allows it to be revenge killed by faster wallbreakers in addition to its poor defensive typing and mid bulk. This means Lycanroc-Midnight needs pivot support, just like the other forms.
Source for what Lycanroc-Midnight does
So, based on their Single tiers, the order going best to worst would be: Lycanroc-Dusk, Lycanroc, Lycanroc-Midnight
Monotype
On the Rock VR, on the SS Monotype VR, Lycanroc-Dusk is C-Rank, Lycanroc is D-Rank, and Lycanroc-Midnight is UR.
In Monotype, do note that the Lycanroc forms are worse, hence their low rankings.
Lycanroc-Dusk: Even though Lycanroc-Dusk has a high attack and speed stat along with Tough Claws, it is outclassed by Barbaracle, Tyranitar, and Rhyperior as physical wallbreakers. Although the three have lower base speed, the former boosts its attack and speed with Shell Smash as well as having Tough Claws, while the latter have better bulk and a higher attack stat.
Lycanroc: Also outclassed by Barbaracle, Tyranitar, and Rhyperior. However, Lycanroc has a small niche of using Sand Rush, allowing you to outspeed fast Pokemon such as Sand Rush Excadrill under Sand, which is why it is D-Rank.
Lycanroc-Midnight: Once again, outclassed by the three Rock-type wallbreakers mentioned above.
TL;DR Lycanroc-Dusk is the best of the Lycanroc forms in Smogon Tiers and Monotype due to a good attacking and speed stats, good coverage, and Tough Claws, albeit worse in Monotype than the tier it is in, UU.
Hope this helps! :)
