***I feel as if ITEMS needs to go first as Viktor has a unique item passive that gives you an automatic item and can't be sold. 6 items will are posted above, but his passive item can not be put up there, so the Doran's Ring gets sold at the end (if the game lasts that long)***

"Hexcore"

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+3 ability power per level / can be upgraded once and can not be sold back or exchanged

"Augment Death"

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+3 ability power per level / +45 Ability Power Augment / Ability: Death Ray sets fire to enemies, dealing 30% additional magic damage over 4 seconds.

"Augment Gravity"

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+3 Ability Power per level / +200 Mana / +10% Cooldown Reduction / +5 Mana Regen per 5 seconds / Ability Augment: Gravity Field has an additional 30% cast range.

"Augment Power"

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+3 Ability Power per level / +220 Health / +6 Health Regen per 5 seconds / Ability Augment: Power Transfer increases Viktor's Movement Speed by 30% for 3 seconds.

I grab Augment Death as the upgrade for the hexcore as it makes you more powerful and gives you a damage over time lazer though this is no the only way to go as you can upgrade to enhance your other abilities.

I start out with Doran's Ring for the mana regen and extra health, as oppossed to an Amplifing Tome, because you have an extra item in your inventory that gives you ability power per level.
Next when I go back I start my boots and if possible I get an Amplifing Tome. Then I finish my boots with Socerer's Shoes. I then upgrade the hexcore to Augment "______" for 1200 (fill in with what you want though I go for "Death"). Then it is time to work on cool downs and get a Morello's Evil Tome. Next you can get the Rabadons for more ability power or you can get a Void Staff if you see them building magic resist (if they are not building magic resist you can skip over the Void Staff and go straight for more ability power with an item like a Will of the Ancients for spell vamp which could keep you laning and fighting longer or other ability power items). Next I get a Linch Bane which will turn ability power into extra damage when you attack after using an ability.

****!!!!!!!Please do not vote yet I will be back to finish and make better!!!!!****

Sorry but I had saved the Items part above and left. I picked up from there when I got home and almost finished and internet had problems and kicked me out :(. Don't feel like typing it all over again and don't think I could type the same thing again. For anything you would like to see (example: laning or team fights) then comment and I promise to come back and update this. Hope the passive and item explaination helps....sorry I will be back.

Also check this site: http://leagueoflegends.wikia.com/wiki/Viktor_the_Machine_Herald
Not all the information is 100% accurate but pretty close hope it helps. If you still are wondering anything or if you think that a different item would be better please comment and I will make this better until this is the top rated guide.