i find ghost and Teleport very important summoner spells because Ghost can help you gank faster and port to turrets that can easily be destroyed ( with a minion there ofc) and allows you to defend your turret.

Dont bother with flash your ult alrdy acts like flash.
If you want to harass ur enemies get clarity but a golem buff can take care of his mana burnings

I go a 9/0/21 masteries to give Kassadin a bit for spell pen and to get ghsot and teleport.

I reccomend to get youer Q first for it silences and can enable you to harrass. At level 4 if you want u cen get 1 point into your W if not follow the rest of the sequence.