Fiddlesticks is an amazing, but underestimated champion. In this build you have the option of taking any lane, especially solo top. He also excels when paired with a tanky hard CC champ in lane. The highlight of Fiddles, however, is that he is a huge asset in team fights! Crowstorm is a giant AoE that puts out giant damage, and allows him to also fear and drain simultaneously. As Fiddlesticks you may not pick up many late game kills, but you be a great asset and assist your whole team!

Take any lane you would like, as long as you are confident in your solo lane abilities. You can even jungle!

Early game:
Take drain first, it will allow you to heal up when you need to. I usually take Doran's Ring, because when you take drain first, you don't need health pots.
This build is all about patience, take advantage of when an enemy champ over-extends; then fear and drain them hard. Depending on the enemy team, you will get at least 1 or 2 kills in the laning phase; even when you solo lane (providing you have competent ganks.) Stay in lane as long as possible and make it a goal to try and grab Sorcerers Shoes and Catalyst on your first trip back to base. Make sure to get Rod of Ages as soon as possible to get the stats up quickly.

Mid Game/Laning Transition:
As the game transitions out of laning phase make the decision of ur 4th item based on the other team's comp. I recommend either an Abyssal Scepter or a Glacial Shroud. At this point there may be team fights, so make sure to keep your ult on low CD by grabbing blue as often as possible. Always Crowstorm after your team has initiated and be sure to fear/drain a scary carry.
When ganking you can also cast Ghost while channeling Crowstorm, this ensures you can keep up to your target and keep them in AoE range.

Late Game:
Provided the other team hasn't surrendered yet, this phase of the game will be centered around teamfights, and therefore, centered around you. You will start to find that you get focused and killed very quickly, the easiest way to avoid this is waiting before you Crowstorm. Make sure you are always aware of where the enemy team is, it will let you stay alive and get into position for sick ults! Remember to focus fear/drain on the most threatening carries.
Once you get your Rylai's Crystal Scepter you will be able to solo Baron; so if you find yourself not knowing what to do, grab the buff and 300g for your team (provided the enemy team has no way of ganking you.)

All in all, Fiddlesticks is a early game lane dominator and a team fight king.

Doran's Ring: Now that the Doran items are 475, you may think that this wouldn't be a good idea, but due to Fiddlestick's epic drain, there is no need for health potions! Mana potions are also not needed because you really shouldn't be spamming drain and you'll only use Dark Wind to last hit.

Sorcerer's Shoes: In case you didn't notice, Magic Penetration is a very important attribute for Fiddles, it allows you to drain and gain health at maximum efficiency, and makes your ult much more effective. This is especialy important for Fiddles due to his low AP scaling.

Rod of Ages: Fiddlesticks core item: it is detrimental to get this item as soon as you can in order to grow the stats as soon as possible. As the first real AP boost in the build, you will notice a significant increase in your drain and ult damage, making Fiddlesticks that much more scary. The health and man boosts also allow for more survivability and more spam.

Abyssal Scepter or Frozen Heart: This is where you need to look at your enemy team and make a decision: If the enemy team is majorly comprised of AP, build into Abyssal Scepter (most cases.) If they are building more AD, build into Frozen Heart. Situational awareness is very important to your success in LoL, so make sure you grab the right one.
Both items provide stat bonuses beneficial to Fiddlesticks (AP and CDR) while providing defensive bonuses bonuses, in this case armor and magic resistance, so taking either wont hamper your ability to succeed.
NOTE: If you're ever in a situation where you don't know which to build because they have equal AP and AD, grab Abyssal Scepter because its passive aura benefits your penetration and friendly casters.

Rylai's Crystal Scepter: This is a great item for Fiddles and many other casters, it provides improved survivability through health and gives that extra punch for your drain, Dark Wind and ult.

Rabadon's Deathcap: The Deathcap is what pushes Fiddlestick's damage over the top. While not necessary during early-mid game, as you approach late game the enemy players may catch on and see how devastating you are and try and stack some MR. How do you counter this? With more AP of course :). If you build to this item your crowstorm will have insane damage and you'll slay squishies with your fear/drain.

Abyssal Scepter or Frozen Heart: Pick up whichever one of these you didn't get before to replace your Doran's Ring, even late game, the stat benefits from either of these items will be well accepted ***ests to your Fiddles build. At this point you're probably about and hour or more into the game and you have the most assists on your team.

Ghost: I personally think Ghost on Fiddlesticks is a must, it allows you to not only get away and catch up to fleeing champions, it also enables you to quickly control the placement of your Crowstorm in team fights and ganking situations. Sometimes it's worth your while to Crowstrom in, pop Ghost, terrify and champ and chase down a carry, leaving the other champ for your teammates.

Flash:

In my opinion, Magic Penetration is Fiddlesticks the most useful and powerful stat. It allows his Drain to hit harder and heal more, it let's his Dark Wind kit harder and finish easier and most importantly, it make his Crowstorm ultimate Crush MR and decimate health bars. For that reason I think these runes are the best for Fiddlesticks.

Greater Mark of Insight X9

Greater Seal of Greater Seal of X9

Greater Glyph of Greater Glyph of X9

Greater Quintessence of Insight X3

As you can see, I take Magic Penetration marks and Quints.