I realize that dark sphere is Syndra's main "most important" ability but it is not worth maxing until late game. This is because AD carry Syndra is only a viable choice because (1). her ult does lots of magic damage even without tons and tons of AP and (2). her Q => E give superb utility, allowing Syndra to stun all enemies touched by dark spheres for 1.5 seconds (which is a lot for anyone, let alone an adc). Maxing scatter the weak is your top priority because at rank 5 its cooldown is a mere 12 (0% CDR) and only costs 50 mana, whereas maxing dark sphere has no effect on its cooldown and doubles its mana cost. Additionally scatter the weak has (in my opinion) the best transcendent bonus, allowing you to stun, disrupt, and re-position everyone in the lane with a single E. As an added bonus, having rank 1 in your Q significantly cuts the mana cost of a 6 sphere ultimate from 400 to 280. After maxing your E, continue on to max force of will. The lower cooldown and transcendent bonus will allow Syndra to continually slow whomever she is kiting/chasing, which will be crucial to pick up kills in the jungle during mid-game. Do not, however, take more than one point in force of will until maxing scatter the weak because the point you took in your W at level 3 will give it a low enough cooldown to continually grab low health siege minions from under tower and throw them at the enemy laner, allowing you to pick up the valuable CS under tower while simultaneously harassing.