You probably noticed that I only advance Kassadin's ultimate at level 6. The reason for that is to have the maximum damage possible with the Q & E combo.

Given that Kassadin is a melee mage and will have to Rift-walk to get close so he can use his combo. So he will need health and the ability to outrun his opponents in order to survive so RoA and Rylais are really useful. If there a lot of stunners in the enemy team then consider getting Quicksilver or Banshee as a last item. If the enemy team has a lot of Magic Resistance then get a Void Staff or an Abyssal Scepter. If they have Massive HP then get a Deathfire Grasp. If you are constantly getting killed by AD Champs then go for Zhonya's Hourglass. If they don't have any of that then get a second Archangel for even more AP.

These spells are my personal choices when I play with kassadin but of course can be switched depending on the preferences of the player. Other good Options are:

1)Cleanse: In case you believe you need more survivability or change the build because it
isn't effective against some of the enemies(A team with a lot of snares or/and
stuns for Example) and find that you are too squishy.

2)Clarity: If you find your self running out of mana a lot then you should consider
changing flash with clarity.

3)Exhaust: You could Also get exhaust although I don't recommend it since your items and
your skills should be enough to kill most foes or escape sticky situations.

Pros: Can escape sticky situations.
Can annihilate any mage. ( Except a good veigar )
Can deal tons of damage with his combo.
Looks like a Bad-***.

Cons: Hard to master.
If stunned while having no items/spells to escape he is dead.
Tough chance you will get to play him in ranked given that they always ban him.
A good talon or veigar will be really tough opponents and probably defeat you if
you don't know how to counter them.