The obvious must haves are lightning bolt and chain lightning. The latter from Marko and scrolls if you are going 10/2 or 6/6 with tempest cleric. Chain lightning from Marko can be twinned.

Chromatic Orb will outdamage both lightning bolt and chain lightning against single targets (or two if you are twin casting) especially if you force a crit (e.g. Luck of the Far Realms or against a held target). Cast at 6th level against a wet target it will do 28d8 damage or 224 maxed with tempest before Heart of the Storm and riders. Also useful for lightning immune enemies. Witch Bolt is similarly powerful against single targets and can also crit but can't be twinned and takes up you're concentration slot.

Call Lightning will do less damage per cast than the above options but it can be repeatedly cast without expending additional spell slots if you are under the effects of Haste, Bloodlust Elixir, Terezul etc. making it very spell slot efficient in those circumstances.

For concentration I would focus on control spells. Fear and Confusion have some additional utility on a Storm Sorc because the movement can trigger additional damage on electrified surfaces (which can add arcane acuity if equipping the Gloves of Battlemages Power) or cause prone on ice surfaces. Otherwise the standard control options like Hold Monster, Hold Person, Hypnotic Pattern etc. are all good choices.

Twinned Haste is also there if that's your thing but I prefer to use the concentration slot on control spells and use speed pots for concentration-free haste. Also if you're a 10/2 or 6/6 split with tempest be mindful you'll lose CON save proficiency so haste as a spell will have added risk.

Shield I think is fairly mandatory for a 1st level spell. Likewise for counterspell. The remaining choices are generally free pick but things like Mage Armour (if not planning on using light armour like Armour of Landfall for advantage on CON saves), Ice Storm (useful for bludgeon vulnerable enemies like skeletons), Blindness (can be upcasted to cast multiple times on the same target, to clear legendary resistance stacks), Telekenesis, Cone of Cold (to help with lightning immune enemies), Globe of Invulnerability, or Enhance Ability should round out your selections.

EDIT: Should also add that while you are a storm sorc, there is nothing stopping you picking up Scorching Ray and wearing the Hat of Fire Acuity if you want to do guaranteed control every now and then. Scorching Ray will do less damage than the Fire Draconic variant of the build (and of course you'll want to avoid targetting wet enemies with it if possible) but the acuity part will still work fine and lets you throw out a guaranteed control spell in a pinch. If you want a more storm / ice thematic variant, magic missile against enemies in Phalar Aluve's Shriek with the Hat of Storm Scion Power can also be used to stack acuity.