Jaina - Default

Level 4

Frost Shards let's you poke through lane minions, summons and objective monsters. It also improves camp and wave clear.

Arcane Intellect gives a buff to your single-target burst when you have a lot of mana, and together with Fingers of Frost gives you incredible mana regen. Consider picking this when you'll be fighting outside of lane without monsters or summons (e.g. Anub'arak beetles or Zagara roachlings) in the way.

Frost Armor is primarily used to ensure any divers are chilled, which makes it easier to turn on them, especially after getting Numbing Blast . Should always be picked into Tracer and Illidan .

Level 7

Icefury Wand adds even more damage to your obscene burst, and improves your camp clear. Also adds some leeway when using Blizzard   for anything that isn't waveclear to get the cooldown back quicker.

Ice Floes can make Cone of Cold almost spammable in fights, and should be picked into teams where you can reliably hit 2+ targets with E.

Level 10

Summon Water Elemental provides strong fire and forget damage, able to pressure enemies and act as self-peel.

Ring of Frost provides incredible burst, and should be picked when there's reliable setup for it (e.g. Blessed Shield, Apocalypse, etc.)

Level 13

Ice Barrier provides an incredible amount of raw stats, making it very hard to poke you, gank you or burst you in fights as long as you hit your abilities.

Icy Veins let's you spam your abilities giving an incredibly strong burst window. Pick this if you're not under threat of dying, or if you need the extra burst damage to successfully kill your priority target.

Level 16

Numbing Blast provides CC acting as self-setup and self-peel, and is generally a very strong pick.

Northern Exposure improves your already ridiculously high burst with armor reduction. You can consider picking this if you feel like you aren't being pressured by divers, your team already has plenty of setup and nobody is bringing armor reduction. Also consider bringing this if the enemy team is relying on armor buffs to survive (e.g. Garrosh , Uther ).

Level 20

Wintermute is almost a must pick if you have Summon Water Elemental , providing an absurd amount of extra damage, making the Water Elemental a very high priority kill target.

Ice Blink is generally strong on a hero that will usually win a fight if she hits a single round of abilities, either providing a safety net or the ability to immediately reposition to follow up on your team's engage.

Cold Snap if you have a very reliable wombo combo with Ring of Frost this adds a good amount of damage.