Hero build eh?
Leviathan:
1st ability, then 2nd ability, then 1st ability, then 2nd ability, then 1st ability, ultimate, 1st ability, 2nd ability, then max out 3rd till you can get the max level ultimate.
Some people would want to raise his 3rd ability first but maxing his first ability will get him virtually invulnerable to creeps.
Then get heart of tarrasque, mask of madness, buriza, and then wateva else.
TB:
Always max out the first and 2nd abilities first, as the 3rd ability is only good when coupled with his conjure image and takes a while to cooldown.
Important note: Do not use sunder while fighting one-on-one against someone. It is stupid. 2 things can result from this:
1. While fighting he spontaneously does a critical on you and kills you
2. You nearly die and out of stress use sunder when both your hitpoints are either very close and after using sunder you see it made no real health difference or even worse when you frantically use sunder only to find you ended up trading your higher health for his lower health and you die.
Skeleton King:
I used to get vampiric aura first but. OK, storm bolt first and then vampiric aura until you've maxed both of them out so you can get critical. It's pretty obvious but, meh.
With Skeleton King, at first you should buy boots of tread and then mask of madness. Pretty straightforward.
For later when your rich, there are a plethora of items that he can own with. Divine rapier, ages of the immortal, refresher orb, heart of tarrasque, and anything else really.