Not Updated For Current Season
I've been playing Kass for a while and almost every build I've seen starts off with a mana crystal and two health pots. Recently, I discovered another build that lets him come out of the gate as a full-out AP carry. Since I haven't seen a good build around this on MobaFire, I decided to make one myself. Comment and rate, please! I'll update frequently and try to improve it.
Flat mana runes are a must for the early game mana pool; other than that, it's up to personal preference.
Levels 1-5 alternate sphere and pulse; once you get your ult, go straight to maxing pulse to massively increase your damage output. After this, max sphere, saving all points in your W to the end.
If you find yourself unable to get blue buff for whatever reason, taking one point in W around level eight is acceptable to make sure you always have enough mana in lane. The mana cost of the active is negligible and the skill is rather useless, so make sure you always pump it when it's active to charge up your force pulse.
Alright here's where my build differs from others. At the start, pick up an Amplifying Tome, a health pot, and head to mid lane. Be sparing with your mana; fire only one or two spheres at level one and try not to cast force pulse at all until you hit four. When you get to level three start harassing in earnest. By the time you hit level six, your mana pool should be completely depleted, the enemy should be at less than half health, and you should have at least 800 gold in last hits. Purchase a Mejai's as your first item, and use Riftwalk to get back to your lane before they know it. Rift in, silence, exhaust/ignite, force pulse for the kill almost every time. Then go to top or bottom lane, and use your bonus AP to net you one or two more kills. Blue pill for mana and buy Sorc Boots. Then, as your foe in mid makes his/her way back into the lane, head down to the jungle and ask a teammate to get you blue buff. This should end all mana problems in the laning/ganking phase. If you have 6 stacks already, you should be able to get a couple more kills and boost it to 10 by the time teamfights start. Be careful though! A death can be a huuuge setback, and with Kass's escape abilities, it should never happen. At the same time, though, don't be afraid to take risks. Kassadin is probably the best tower diver in the game.
Midgame, you will need more mana in your pool than the slow regen of blue buff can provide. Though you should still keep the buff on you at all times, it's generally good to get a Tear of the Goddess and begin building your pool. Teamfight as you normally would; there are many other guides here that give great mid- and late- game guides. If you are feeling a bit squishy and want to be extra cautious not to lose your stacks, grab a Giant's Belt and build into a Rylai's. If you want to one-combo Ashe and Tryndamere, get a Large Rod into Rabadon's. If they are MR tanks, get a void staff, and if they are HP tanks, a Deathfire is the way to go. Any time you bluepill with less cash than you should, finish your Archangel's.
One other thing I have tried a couple of times, which seemed to work well: If your team isn't playing well but somehow you manage to be carrying anyway, it can be helpful to purchase a dagger or a zeal. This massively increases your ability to push towers, and when they come to try and stop you, just hop over a wall into a nearby jungle and teleport on your merry way to the other side of the map, where you repeat.