Not Updated For Current Season
Try getting some magic penetration marks, ability power/level seals, ability power glyphs, and health/level quintessences
This mastery build is something I did, just so Fiddlesticks can get a small boost of AP and spell penetration, along with Flash cool-down reduction so you can use it with Crowstorm a lot more often. I also have a point in Cripple so you can use Exhaust on those with higher magic resist to take them down a faster.
This is what I do for his abilities, but I really don't have a preference. Build it how you feel would be best while getting spell vamp items.
Exhaust and Flash go best with this build, mainly because you can hit higher magic-resistant champions with Exhaust (Magic resist is reduced with the Cripple mastery) and use Flash to get up to fleeing champions while you have Crowstorm going.
Start out with Doran's Ring. Its the best for dealing a fair amount of damage, mana regen, and health when starting out. Start working on Sorcerer's Boots when you have the gold, then start working on Hextech Revolver (You don't have to get Will of the Ancients immediately). Now start getting Rod of Ages to boost up your mana and health (You may swap this for Ryali's Crystal Scepter if you need more survivability or swap it for Archangel's Staff if you're doing good). Continue on by getting Abyssal Scepter for an extra magic resist and magic resist reduction to nearby opposing champions (Which comes up to a total of 30 with Fiddlestick's passive and 40 to one unlucky player who gets hit with Exhaust). And finally, start by getting Zhonya's Hourglass for some armor and a large ability power boost, but you're mainly getting it for its activate, which can be used during Crowstorm. If you want to step it up, you can sell Doran's Ring and start working on Rabadon's Deathcap for extra magic damage.