Rankings

Primed Target Cracker - +110% Crit Damage for Secondaries. Must have for any Crit Secondary, which is a lot of them.

Primed Regen - allows Sentinel to revive up to three times when destroyed. Useful for keeping the otherwise rather squishy Sentinels around longer in higher levels, however runs out quickly in very high damage content. Note also that Sentinels are being reworked to auto-revive (announced during Tennocon), however the exact mechanics and this mod's interaction with them are still unknown.
Prisma Shade and Prisma Burst Laser - Sentinel and weapon. Outclassed by its Prime version, however notable as a great source of Mastery (6000 for Sentinel, 3000 for weapon).
Void Trader - colour palette that notably can be acquired without spending Plat.

Prisma Twin Gremlins - MR11 Dual Secondary. While solid, it is largely outclassed now by a number of other comparable weapons (e.g. Kuva Twin Stubbas, Tenet Diplos).
Primed Sniper Ammo Mutation - Ammo Mutation mod for Snipers. Often overkill as regular Ammo Mutation mods are usually sufficient, notably with Vigilante Supplies marginally buffing Crits as well. Notable for being 0 drain when unranked.
Sands of Inaros - Quest for Inaros. Always in Baro's inventory.
Baro Void-Signal - Key to exclusive mission line, awarding cosmetics. Can only be purchased and played with Inaros Prime. Always in Baro's inventory.

Vermillion Storm - Claw Melee stance. While decent, it can be acquired from other sources (notably Kuva Guardians), and can also be bought from other players for very cheap.
Opticor Vandal - MR14 Charge Rifle. Decently strong single-shot attack, with a small AoE. Can be a little inconsistent with proc output, particularly Slash procs when relying on Hunter Munitions. Notably, can be farmed from the periodic Thermia Fractures mini-event, which you should also do for access to the Exploiter Orb fight for Hildryn parts (and resources).

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