Rankings

Jolt - 60/60 Electric mod for Secondaries. Essential for any build that wants Status Chance with Electric/combined-Electric damage. Can only be acquired from Baro (or other players).
Shell Shock - 60/60 Electric mod for Shotguns. Just as important as Jolt, except Shell Shock can be farmed from Hive Sabotage on Eris at a low drop rate. I wouldn't bother as the drop rate is very low.

Prisma Grinlok - MR11 Semi-Auto Rifle. Slow-firing, with decent stats and notably high Slash bias. While not bad in a vacuum, it is a single-target weapon in a game that is largely a horde shooter, and has to compete (unsuccessfully) with much stronger weapons like the Kuva Chakkhurr and Latron Incarnon.
Glaxion Vandal - MR12 Beam Rifle. Outclassed by its brand new Tenet version.
Primed Cleanse Corpus - 1.55× Damage to Corpus for Shotguns. Strong for endurance runs, but largely overkill in the vast majority of content.
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.

Maim/Fanged Fusillade - 120% Slash mods for Secondaries/Rifles respectively. Both have niche uses at best, and both are farmable from the Hydrolyst, or buyable from players for quite cheap.
3 Day Credit Booster - Buy it from Market for Plat instead.

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