“Must Have” being hyperbolic, obviously since you can totally play without any but that’s not really a helpful answer to what you’re looking for.

Before getting into “what arcanes are good” I think it’s important to understand “what role do arcanes serve”. Right now warframe is in a phase of its life where builds are all about outsourcing. Finding a build you want that needs to hit a certain breakpoint, or has a certain weakness, and seeing if you can get those stats from some other source, or cover up your weaknesses. Below aren’t all the good arcanes in the game, but they’re ones I’ve used regularly or think are worth looking into because they do just that. They allow you to change how you play, and have more fun with build freedom and flexibility.

Warframe Arcanes:

• Arcane Energzie: Yes it’s good, but yes it’s expensive. It comes out to about 10 energy per second at max rank, I’ve been around a long time and I don’t have a maxed out one, I’d like to some day but energy restoration is less of a big deal these days.
• Arcane Avenger: +45% final additive critical chance. Ie, if in the mod screen you have 55% crit chance, when Avenger is up it straight bumps you to 100%, very useful on a lot of builds that use Combat Discipline to self damage and trigger it.
• Generic Eidolon Weapon Arcanes: Things like Fury, Strike, Acceleration, Rage and Precision. None of them are ones I’d go for first, but they can be useful for allowing you build freedom and diversity.
• Zariman Molt Augmented and Molt Vigor: Both offer final additive strength bonuses. Augmented ramps up and is better suited for things that take longer than an exterminate, but can get you +60% strength, but once you build it up you have it til you die without a timer. Vigor requires an operator cast and only works for a single ability but is +45% strength and is better suited for abilities that can “snapshot” warframe stats, like Gloom or Vex Armor. Abilities where when you cast it, you can either recast it and keep the original strength or it’s drain based.

Weapon Arcanes:

• Primary & Secondary Merciless, Deadhead and Dexterity: All of them are cheap to pick up once you have access to weapon arcane adapters, and they radically change how you’re able to build your weapons. They functionally all do the same thing with different minor bonuses and different trigger conditions. Merciless triggers on any kill, even DoTs. Deadhead triggers on headshot direct damage kills so not bleeds or burns etc. Dexterity triggers on melee kills so is best suited for a gun you only want to pull out periodically.
• Cascadia Flare: Only applies to secondaries, is one of the nicer weapon arcanes right now. Unlike the previous ones that trigger on kills, Flare triggers on status effect, from any source. As long as you’re applying heat procs your secondary will be ramped up, and if it’s a high status chance weapon it can even ramp itself up without struggling to kill early enemies.

Operator Arcanes:

• Magus Elevate: If you’re worried about dying and you can survive hits, ie you’re still relying on health and armor, elevate is basically infinite free healing, as long as your frame isn’t getting exploded elevate will heal you back to full pretty much for free in no time.
• Magus Lockdown: Causes you to CC a clump of enemies by void slinging at them. Works on Demolysts for Disruption missions. Huge amount of survivability on any frame loadout, and can root enemies so you can line up headshots for incarnon battery charging regardless of what frame you’re bringing.
• Magus Anomaly: Pulls in all enemies in a radius whenever you transfer back into your warframe. Offers loose grouping on literally any warframe. Can be useful for AoE weapons, but also for things like Khora’s strangledome or Caliban’s armor strip field. It allows you to for free pull enemies anywhere you want them to be, it’s not as instant as something like a grouping ability like airburst but there’s no cooldown.

Amp Arcanes:

• Eternal Eradicate and Eternal Logistics: Best in slot, hands down. They give your arcane a huge amount of consistent crit chance and crit damage, all you need to do is use your operator abilities.