Why? Because i got him and I'm going to crush every one. Basically, he's one of the most annoying and strongest ARAM champion.

flash:it can save you, or kill someone else. But basically saving your teammates. OwO
heal:for early game survivability, or movement speed. also, saving teammates.
ghost: doesn't help much with ult, so it's useless
barrier: never get it. the dmg it can take is so tiny comparing to your w
cleanse: another useless one
clarity: u never need it
ignite: only when thirsty for first blood
exhaust: if you're going more a support
mark/dash: depends on your playstyle

well, basically, you're a tank, so every defensive item works, depends on play style and enemy. (warmog is usually a must get) personally i get rod of ages if they don't have much poke

passive:you get anther few seconds to do what you haven't finished, best ability for taking down turrets(especially if you dare to go ad ;)
q:just give 1 level, all you need is the cc
w:amazing sheild and passive, even better than nasus' q and veigar's q. it gives you max health on kill, no matter what ability. max this third.
e:strong poke, gives cc and dmg, max this second.ALWAYS HIT THE MINIONS, it gives more dmg, and you have multiple targets and high range
r:tons of cc and dmg, just super strong, max this first. if you can, walk back a little then charge in for higher dmg. DO NOT WALK TO THE VERY END. HIGH CHANCE THAT YOU WON'T MAKE IT TO THE TEAM FIGHT.