With Summoner Spells, Flash is a must as with most champs, but the second spell is really up to you. Teleport is good for roaming and staying in lane longer, Ignite or Barrier can be useful in fights, and maybe Exhaust or Cleanse can come in handy for certain match-ups, but I generally prefer Ghost for this build as it provides an engage tool which basically allows you to get a guaranteed Event Horizon around an enemy or chase someone down to finish them with Primordial Burst, and it can also get you out of sticky situations when you don't have Flash up.