Hello everyone. I felt the need to do this since unlike Runes, Relics get new additions once in a while, and it'd be a chore to update every single one of my analyses just to add said Relic to my list of options. Instead, I will analyse every respective Relic here and tell in which situations or on which kind of monster they'd be useful on so that you may experiment more freely and I may relieve myself of the need for updating my past analyses.
Before beginning, I must mention that some Relics are obviously weaker than others and some Relics are not useful at all. To more effectively show this, I will list the higher powered Relics before those inferior ones so that you can know the one below is weaker than the one above, but can sometimes be used as a substitute if you don't have access to the latter.
Masks generally offer one of three things: dealing damage to enemies under a certain condition, Stamina control, and Strength boosts. This makes them a flexible slot that can be used by attackers and more supportive monsters alike.

Jasastur's Mask: This is one of the strongest Relics in the game. Not only does it drain Stamina from the target(s), but it also restores the wearer's Stamina. Any monster would enjoy this Relic, but especially good candidates would be extra turn spammers or monsters that burn through Stamina quickly.

Talany's Mask: This Mask is a simple but powerful option, especially when combined with other Armor giving Relics like Atum's Armor and Tenacity Shield. It helps cushion the assaults of attackers and Torture effects, making it a generally useful pick.

Eeltron's Mask: Eeltron's Mask is an excellent pick on tanks to deny the attacker for its next turn while giving the holder protection should their Life drop too low. You can still use it on other monsters too though, since it discourages attacking the holder in general.

Baltasar's Mask: If you're using this Mask, it means that you're planning to wipe out the enemy in a single strike, or take down an important target in one hit. Use it only in this situation, as otherwise you'd be wasting a valuable utility slot for a plain damage boost.

Fury Mask: This is an inferior Jasastur's Mask that doesn't drain Stamina, and it checks for the enemy's Life rather than Stamina to activate.

Exhausting Mask: Inferior Eeltron's Mask.

Healing Mask: Mostly an inferior Eeltron's Mask unless you face Pierce users or have a strategy that revolves around real healing over the pseudo-healing of Armor.

Thunder Revenge Helmet: Skip.

Energy Mask: This Mask is useful versus Stamina Draining monsters to avoid being denied. You can use it in a battle where the opposing denier is a Stamina Drainer like Wangzhou.

Great Enraged Mask: Inferior Baltasar's Mask.

Despair Mask: Skip.

Draining Mask: Very inferior Exhausting Mask.

Charge Mask: Inferior Energy Mask.

Mask of Darkness: Inferior Thunder Revenge Helmet.

Enraged Mask: Inferior Great Enraged Mask.