Hello there! My name is Jay, I've been Diamond on EUW for over three years now. I have over 100k points on Alistar with a 70% ranked win rate. If you wanna know how to carry hard with a very simple champion, this guide is for you. Enjoy.
Flash is easily the best summoner spell in the game. Even if it wasn't there would still be the argument that Flash synergises extremely well with Alistar in general for multiple reasons. The Flash - Q - W combo being one of them.
Ignite is for kill pressure and heal countering. Exhaust is possible as well, although much worse early game.