Not Updated For Current Season
This build was taken from Pedel's guide posted on leaguecraft.com / leagueoflegends.com . I did not create this build, just posted it here in build-form.
This build maximizes Fiddle's killing potential by utilizing the powerful combination of terrify and drain. The summoner spells flash and drain also increase Fiddle's kill count, both allowing him to get in range of enemies for drain or his devastating ult. Thank's to Fiddle's passive, which reduces the magic resistance of nearby enemies, and this build's masteries, which decrease cool-downs, you can use your runes to focus on ability power, which makes Fiddlesticks extremely volatile.
For the item build, you may want to get things out of order. After your sorcerer's shoes, you need to determine wether or not the opposing team has a good interrupt (i.e. one that can cancel out both your drain and ultimate.) If they do, then buy banshee's veil. If not, you can skip banshee's veil and buy the rest of the items in order. If the enemy team also has extremely high magic resist, you should buy the abyssal scepter first, before continuing with the build.
For a more in depth look, including a flowchart of what items to buy, check the original build here.