Fiddlesticks is one of the strongest champions at Low ELO. His Bountiful Harvest allows him to beat most champs 1v1 [or even 1v2], his Terrify can completely disable a person and his ultimate Crowstorm is just amazing. In my opinion, Fiddlesticks can beat anyone without a stun or silence or knock up easily, in lane and in fights.

For nearly twenty years, Fiddlesticks has stood alone in the easternmost summoning chamber of the Institute of War. Only the burning emerald light of his unearthly gaze pierces the musty darkness of his dust-covered home. It is here that the Harbinger of Doom keeps a silent vigil. His is a cautionary tale of power run amok, taught to all summoners within the League. Decades ago, there existed a powerful rune mage from Zaun - Istvaan. At the end of the fifth Rune War, he became one of the League's first summoners. Too much a prisoner to the old ways of magic, Istvaan stepped further and further outside the rules of conduct in the League. In what was ultimately his last match, his reach finally exceeded his grasp. Sealing himself inside the easternmost summoning chamber, he began incanting the most forbidden of rituals - an extra-planar summoning.

What exactly happened inside that chamber remains unknown. No champion came to represent Zaun that day in Summoner's Rift. Only silence echoed back from repeated knocks on the chamber door. The first apprentice who entered was cut down immediately by an unearthly scythe. What few who followed and survived were driven mad by fear, mere husks of men gibbering about crows and death. Afraid of the evil even Istvaan could not control, the League sealed all exits to the chamber, hoping they could contain what they could not destroy. Years went by, but the wooden figure within never moved save to slay any foolish enough to enter. Seeing no recourse to reclaim the chamber, the Council instead devised a use for Fiddlesticks: executioner. While he comes to life and seemingly abides by the rules of summoning in the Fields of Justice, what he awaits inside his chamber is unknown. His unmoving face yields no clues, and his scythe stands ready to strike down any who stand before him.

Those who say 'you have nothing to fear but fear itself' have not yet felt the crows.

Greater Marks of Insight gives a nice amount of Magic Pen
Greater Seal of Replenishment helps Fiddle's mana problems
Greater Glyph of Scaling Ability Power gives a nice amount of AP
Greater Quintessence of Ability Power gives a nice amount of AP

I go 9/0/21 for the 15% Magic Pen in Offense. Next the Utility helps a lot with the mana problems, lower cd Flash, faster leveling, longer buffs, etc. These are probably the best masteries you can take as Fiddlesticks as Defense isn't as important.

Doran's Ring start gives a nice amount of AP, health and Mana Regen. This is by far the strongest first item that Fiddlesticks can get.
Next I get Boots for a little bit of movement speed and rush Hextech Revolver for unbelievable sustain and a bit of AP.
Afterwards, a Mejai's Soulstealer gives a massive amount of AP when stacked. Being Fiddlesticks, stacks should come easily with ganks.
Rylai's Scepter next, not just for the nice slow, but mostly for the survivability it gains [HP+AP]
We get a Rabadon's Deathcap next. If you have a decent amount of stacks, you should see a notable increase by the 130% AP Passive. The +140AP is also very nice. After this we don't really need much more AP and will start to go for Magic Pen.
It becomes a bit situational. If they are stacking MR go for a Void Staff. If they aren't, an Abyssal Mask is very good. And we finish with a Zhonya's Hourglass for the epic activation and the nice amount of Armour and AP.

Crowstorm > Bountiful Harvest > Terrify > Reap
Drain is his bread and butter and should be maxed ASAP. We put 1 point in Dark Win at Level 4 and keep it like that as it is only used for the silence. After Drain we level up Terrify. We pick up Crowstorm whenever possible.

Fiddlesticks is good in either a solo or duo lane [with a person with CC]. If duo lane, head for a level 1 fight in the brush as Fiddlesticks is unbelievably strong early game. Whether duo or solo, fiddlesticks play the same. Farm, farm,farm. Terrify + Drain enemy. The Terrify + Drain can easily drop half the enemy's hp. As soon as you hit level 6, run into the brush and ult in. If you can't ult him [he is outside of ult range/scared], then go gank another lane. Once you ult in quickly use Terrify + Drain. This should be insta kill if done properly.

Fiddlesticks job in teamfights is simple. Ult in with Crowstorm, QW their carry and hopefully murder the whole enemy team. If you ult and the enemy team runs out of range [without them knowing] don't just give up. You can flash into them and still do everything. Once you finish your ult and QW, quickly use Zhonya's Hourglass. Remember to always have blue whenever possible.

Fiddlesticks is an unbelievably strong 1v1 fighter and can easily destroy uncoordinated teams. His ultimate can change the game and win teamfights.