Viktor Syndra or Cassiopeia ? (please help)

Depends on what you want. Vik has waveclear and AoE. Syndra has a lot of neat combos and great single target dmg. Cass is queen of DoTs and has really good lockdown with her Grounded debuff and ult.

As a heavy mid player I can tell you one thing. It is situational. Each of them has their strong points and weak points.

High waveclear and zoning power. High burst and AOE damage. Highly technical, usually takes longer to learn.

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High Dps. Grounded status prevents movement abilities. Tends to have mana problems early game.

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Strong AOE. Can move more freely as her basic spells have no casting time. Has very high range stun.

Each has their own good and bad matchups, if you're talking picking blind it really comes down to your team comp and comfortability.

Cassiopeia can single handedly carry games with ease. Highest DPS in the game. Bad vs. mobile champions with strong gapclosers (Diana) Melts tanks. Good waveclear.

Syndra is a burst mage with AOE damage, but is best at eliminating a single target. Long range stun, very strong at level 18 because of passive. Struggles against assassins like Talon or Akali. Very good waveclear - high mana cost W.

Viktor is a slightly mobile mage with very high damage in the second hit of his E. I don't know that much about him to know what he's bad/good against. Very good waveclear.

I'd go with Cassiopeia, she has a defensive tool (Her ult) that can also be used to suprise someone when they facecheck a bush. She's easy to use, and doesn't really need her ult to kill someone. Farming under the turret, and just farming in lane is really easy with her because of her E. She also isn't as hard to use as Syndra or Viktor.

:^)

viktor is the safest really

Syndra if you want best burst, cass for best sustain damage in the game, viktor is safer and a bit burst but sustainy too