There is no other option to consider. Even with Kassadins ult, flash is still a must have in all situations. It allows you to survive your vulnerable early laning before level 6, in which you are very easy to gank. Flash increases your range of threat which is already so high because of Riftwalk, as at any time you could Flash Riftwalk and use your combo. This forces opponents to either play a lot safer, or risk being caught from such long ranges. It can also be used to be in range to use your Riftwalk for damage, which is why you should always Flash->Riftwalk when using it aggressively. If you are not getting Zhonya's Hourglass, or have not yet built it, you can use flash instead to engage and bait teamfights. Riftwalking forward then using your combo to burst down a target and slow their team, then flashing away before being locked down and killed can allow your team to catch them and force a favorable fight.

Assassins need ignite to add to their burst and secure kills. Ignite is so vital for Kassadin as often times, before getting items, his damage is not high enough to secure kills without it. You will find during the early/mid game almost all of your kills will be due to ignite. Late game this summoner spell will allow you to burst down AD carries and not allow them to quickly regain that health via life steal or Blade of the Ruined King. This makes you a great duelist even with AD carries as even after you have used your full combo, your dps is very high with your activated w and ignite. Some people like to take teleport, but I think ignite is just too important in getting you to snowball from early-mid game kills.