Why is Alistar one of the best champs?
Probably, because he has the possibility to change a whole game and to lead your team to the win. With his abilities he is extremly strong in jungling, as specially in ganking. With his headbutt and pulverize he can destroy each lane and help the laners to a better cs and advantage against the enemy.
In teamfights he is incredible hard to beat, because with his ult and other abilities he can tank for hours and assist the carries while killing the opponent.
So if you know how to play him right, and you don't fail the early game, you can win each game (even a solo queue with a lot of green players).

First, why do I recommend these runes:
The quintessences and marks support the strong abilities of Alistar. In combination with later items the bonus ap and mp is extremly good. It also helps you in the early game, while jungling and ganking.
The seals of armor and glyphs of magic resistence support the tanky part of Alistar. As specially in the late game you can get really tanky and stay in teamfights for long times.

THE BEGINNING: "GET FASTER"
First you should start with boots, because Alistar isn't the fastest jungler, so you really need the movement for a faster way through the jungle and better ganking success.

As second and third item you should be the two gold items philosophers stone and heart of gold because with the gaining extra money you can buy further items faster. (Philosophers stone is also good because of the mana and health regeneration)

Next you should buy the boots of mobility because afer owning them you are so fast that you can destroy everything and paricipate at each team fight.

THE CORE ITEMS: "SUPPORT YOUR TEAM"
If you want to have a strong and undestroyable Alistar you need two core items.
They are:

Shurelyas Reverie:
This item gives you mana and health regeneration, cooldown reduction and a powerfull active ability. As I show later, this active bonus movement speed is the core of Alistars power. If you use shurelyas at the right time you can change a hole game with it.

Aegis of the Legion:
This item, in combination with Shurelyas, make Alistar becoming a true support tank, who has the ability to start teamfights and survive for the hole game. With its armor, magic resistance and health Aegis provides everything a good tank needs and its Aura supports the power of the Carry.

Oracle's Elexir:
You need the oracle to destroy the enemies wards and to help your team in teamfights. Because a very important factor in the gameplay is the map control, and a team without any wards has nearly no chance to win. Also you should buy it after Shurelyas and Aegis, because now Alistar should be tanky enought to survive each teamfight, so the oracle doesn't goes down.

THE LATE GAME: "DESTROY THE ENEMY"

IF EVERYTHING IS PERFECT: "THE TANK"

Abyssal scepter:
If you own this item

IF YOUR BOTLANE LOSES: "GET ARMOR"
IF YOUR MIDLANE LOSES: "POWER OF ALISTAR"

THE FIRST ROUTE: "SMITELESS BLUE"
If you want a perfect start in the jungle do the following: Start with wolves and use pulverize one time. Use the first health potion while fighting the blue. Here you should ask your team for a lot of assist, because it is usefull to safe smite. (For example if you play a team ranked game, your laners can help you, in solo queue they often don't fight at blue as long as you could need them)
After killing blue without smite, go directly to your red and smite him.
Now you have to possibilities:
If you see that there is a possible gank on bot or mid lane, do it, so you can achieve first blood for the laners. (This will give an excelent start for the lane)
If there is no chance for a good gank, stay in your jungle and take the wraiths or golems. Farm as much as possible (you can also try to go in the enemy jungle and steal something there ;)
In the following time you should reach level six as soon as possible and try to use every chance you have for a successfull gank, because this will help the laner incredible to a hudge gold advantage.

THE SECOND ROUTE: "HELPLESS AFTER BLUE"
You do the wolves and start with blue, but there is no one going to help you. Use smite so you don't get damaged too much. Afterwards go to wraiths and golems, leave your red until you have smite again.
If this is the case (and it will happen in nearly each solo ranked game) gank any lane after you are level three and do the lizards after you had a successfull gank.
GANKING "THE ART OF W-Q"
TANKING "THE ULT"
THE LATE GAME "USE SHURELYAS"