First of all, give http://www.leagueoflegends.com/board...ead.php?t=2619 a quick read before continuing on here. It lays an excellent foundation and describes Fiddlesticks' skills (which I won't do here). This guide is being written assuming that the reader has a basic understanding of skill builds, items, abbreviations, and the general mechanics of LoL.

Skill Build:
1 - Dark Wind
2 - Terrify
3 - Dark Wind
4 - Drain
5 - Dark Wind
6 - Crowstorm
After 6:
Crowstorm > Dark Wind > Terrify > Drain

The build I (and the other cited guide) advocate is a quick Dark Wind build. Secondary to that, Terrify is gold for just general utility. A single point in Drain early on allows you to use it; it performs functionally as HP replenishment fine at 1 rank and tends not to shine offensively until it has a lot of AP behind it.

Items:
Doran's Ring, Health Potion at start
Boots of Swiftness
*Mejai's Soulstealer
*Lich Bane
Abyssal Scepter
*Warmog's Living Armor
*Banshee's Veil
Zhonya's Ring

All items marked with * have a situational use, see below:
-Doran's Ring has proved itself time and time again to be an incredibly useful starting item, despite a few other potentially better-looking starting items, so I've stuck with it. Health Potion for early lane longevity.
-The natural boots choice would be Sorcerer's Boots (especially since you'll be wanted lots of spell penetration as described below), however the Movement 3 on Boots of Swiftness is practically a requirement to get the full bang for your buck out of Crowstorm. People ARE going to run from you, and you will need to be able to keep up.
-The new Mejai's Soulstealer is a fickle item. In order to get it to an acceptable AP/gold ratio it needs quite a few stacks. I only get it in solo que games where I take the solo lane; ie use it in games where you can be sure you'll have a foothold from which to gain a lot of kills (at least over the course of the game) as soon as you buy it AND if you're good enough not to die too much to impede buff stacking.
-Lich Bane is a potential early alternative prior to Abyssal Scepter. It offers a number of advantages (most markedly, the % movement speed increase) at the cost of a rather hefty pricetag. Nowadays this is my Mejai's Soulstealer replacement of choice when I play with my regular group; even when I take solo lane they're all good enough gankers that I usually end up with a ton of assists or never making it to the gank before the enemy is dead.
-Abyssal Scepter is required. AOE spell penetration builds on Fiddlesticks' aura and makes Crowstorm that much more deadly.
-Warmog's Living Armor if you have, or suspect you'll have, any substantial difficulty staying alive through Crowstorm and/or team battles.
-I've been starting to shy away from Banshee's Veil lately; in my opinion most of the situations it would theoretically be useful are trumped by Warmog's. Keep it in mind, though.
-Finally, stack Zhonya's Ring until you're out of inventory spaces. Don't forget to pawn off your Doran's Ring for an extra Zhonya's if you run out of space too. Buy this extra early if your Mejai's starts getting really high stacks to capitalize on the AP+25%.

Runes:
Right now I would recommend that your rune choices focus on spell penetration, cooldowns, and ability power in that order. Right now I'm using Blue, Red, and Purple to stack spell pen and Yellow to stack cooldowns.

Masteries:
Offensive:
3/3 Archmage's Savvy
1/3 Deadliness
4/4 Sorcery
1/1 Archaic Knowledge

Utility:
3/3 Perseverence
1/3 Good Hands
4/4 Expanded Mind
1/1 Greed
3/3 Meditation
3/3 Quickness
1/4 Awareness
3/3 Intelligence
2/4 Awareness
1/1 Presence of the Master

The above is my recommendation. Feel free to fudge points as per personal preference or for summoner spell buffing.

Summoner Spells:
As of current patch I recommend Teleport and Cleanse. Before the current patch it would have been Flash instead of Cleanse, but the survivability advantages Flash had before it was nerfed are now watered down to marginal. I tend not to use Flash offensively, but if you wish to use it with Crowstorm to shut down fleeing players it'll still do the job.

Strategy/Micromanagement:
I'm not going to get into the intricacies of dealing with individual heroes/team compositions--the finer points of which strategies rely heavily on experience, and deal in a few personal trade secrets.
-Dark Wind doubles as an excellent creep farming ability and an effective harassment tool. It will also destroy groups of neutral creeps. Avoid targeting players directly unless they are within immediate range.
-Never casually blow Dark Wind on a lone enemy or creep.
-Terrify=>Drain is an excellent combo, but don't be afraid to use either separately.
-Astute opponents will do whatever they can to force you to break Drain once they catch you using it, keep this in mind.
-Whenever possible, always begin Crowstorm from out of enemys' sight. This can be done over jungle walls or from grass. Failing that, make sure you have some distraction.
-Harass melee heroes who move up to last hit within range of your attack.
-In the early lane phase, a good Crowstorm will destroy opponents who stick around at low to medium health. Make sure they won't have time to retreat to their tower if you go for it.
-If able, Dark Wind and Terrify both discourage enemy pursuit.

Probably the only Fiddlesticks guide i approve of. And I only play fiddlesticks for the most part anymore.

My only differences which you might consider:
-High Elo ratings you need a Rod of Ages, otherwise you are far too squishy. Depending on the team, i may have two. Warmogs is a great HP item, but RoA gives a more well rounded benefit in addition to a Catalyst the Protector to level with
-Yellow runes are probably best used for AP runes. A more often crowstorm is a lot less useful and a more powerful one.
-As with every character, I firmly believe any game that you think will last any bit more than 60 minute, you need a Guardian Angel. 90 seconds of dead time is far more detrimental than 100 AP less.

These are all great points, but let me play devil's advocate for a few:
-For 70 more gold Warmog's gives you 650 more HP (given both items at max bonus) plus up to 18.5HP/sec regen; while a Rod of Ages has extra mana and AP, strictly speaking neither of those contribute directly to surviveability.
-Full Tier 3 AP/level yellow runes gives 16.2 AP at level 18. Full yellow cooldown runes knocks a whopping 3 seconds off of Crowstorm. In my opinion, no matter what you rune for it's going to work out to be a disgustingly minor boost such that it's hardly worth arguing over (not that I'm adversed to minmaxing); tis really personal preference/placebo effect.
-Guardian Angel doesn't guaruntee you won't be reganked once it kicks in. This would almost be a great argument for Revive if there weren't a ton of readily identifiable trade-offs you're making in summoner spells. Plus there's better ways to get armor (Frozen Heart) and MR (Banshee's Veil) if you need tons of one or both.

Don't get me wrong those are great item options, just that, as with anything in League of Legends, it's helpful to be aware of the trade-offs you're making.

Yea, i see what you are saying on those points.

RoA is built off of Catalyst, obviously, which is a great leveling item. In games that i get one of, ill end up with around 2700 hp, in games with two, over 3k. I think anything more than that might be overkill at the expense of AP.

With CD reduction, i generally group with an amumu, and we attempt to match CDs as much as possible. So its moreso him matching with mine.

Of course you could always be reganked, but if you are ever not in a team fight (aside from backdooring) at the hour+ mark, you are doing something wrong. 3-5 Guardian Angels will pretty much guarantee your team will not be the one at the 90 second respawn.

Obviously, everything is based upon personal preference, play style, and who you queue up with. Not every fiddlesticks has the luxury of laning with an Amumu everytime like I do, so your build might be more suited to someone solo queueing.