This guide will be showing you how I play Fiddlesticks on almost every game. Whether it be a 5v5 or a 3v3, this build will get you a win as long as you're smart. Let it be known now, the item build is not set in stone. There will be many times where you will need to mix the build up a bit in case you have some guys packing MR. This guide is really effective against squishy characters just because of the amount of damage dealt and health acquired early game. As long as you don't feed the enemies, you'll carry the team most of the time.
As said in the introduction, the items may have to be rearranged or replaced occasionally with different items. However, there are items that must be set in stone to have a good early game attempt at making the other team go "WTF?!" First off, the Doran's Ring and Sorcerers Shoes are a must. After you acquire your Sorcerer's Shoes, get around 1K, then sell your Doran's Ring and get the Hextech Revolver. This handy little item will give you 15% spell vampirism ON TOP OF your current lifesteal. At one point it was 20%, but in a patch over the summer it was nerfed to 15%, which brings the next item into play, the Will of the Ancients, which brings your lifesteal up to 25%, and also gives your nearby teammates +30 AP and 25% lifesteal too. This will cause you (and your teammates) to steal much more health from your enemy, thus making them very angry if they decide to turn around and attempt to fight you. Of course, if they have some sort of CC, you'll probably get screwed over unless you have a teammate backing you up. Getting a Rod of Ages is also a must, since it gives you more AP as the game progresses.
DEVELOPER COMMENTARY ON PAST BUILD
(skip this if you don't wonder why I didn't include WotA in the build in the first place):
A lot of people commented asking why I didn't put it into the build when I first made it (when hextech was 20%), and that's because it was such a small increase in both lifesteal (5%) and AP (10), that I didn't see the point of wasting 900 gold on it rather than using the gold for the Blasting Wand for RoA (40 AP). If we do the math, say you have hextech and blasting wand as your only items. You'd have 80 AP and 20% lifesteal, and AP is directly proportional to how much damage you do, so 20% of 80 is 16. Now if we do the other scenario, where you have 25% lifesteal and only 50 AP because you used the Blasting Wand gold on WotA, you'd only get 12.5. I realize that if you had a mostly AP damage team that could use the extra AP and lifesteal to an advantage that you'd lean more towards getting it but normally I'd be on teams where we had a tank, 2 DPS, and 2 AP.
The next items you get after you have a WotA, Sorcerer's Shoes, and Rod of Ages are completely arbitrary to how the game is played out. If they start stacking heavy MR, then you may want to get an Abyssal Scepter before a Rabadon's to penetrate it, or if they keep screwing up your drain cycle with CC, you may want to invest in a Banshee's Veil, etc.
There may be special occasions where you will want to replace the Abyssal Scepter with a Void Staff. One of these occasions is where you get an extremely tanky enemy team that's stacking a LOT of MR, and are focusing their items mostly on keeping you from dealing damage so that your lifesteal will be infective. This is one of the times a Void Staff is wanted rather than an Abyssal Scepter because it will lower their MR more so than an Abyssal Scepter (If the majority of their team has >50 MR, then get this, as 40% of 50 MR is 20, which is the amount Abyssal Scepter gives. Also, since they are tanks, you shouldn't need the MR of abyssal scepter, as they should deal less damage).
You may be wondering, "If Fiddlesticks is so squishy, why not get him items like armor, health, or MR?" Well, if you get enough AP, you don't even need that stuff, because Fiddlesticks drains life so fast with not only his drain, but also his dark wind and ult. Therefore, the only time you'd be screwed, is if you were ganked, or if you initiated and your team didn't follow you in.
Runes are completely arbitrary, so I'm just giving you what I use. I like having ability power more so than magic penetration because Fiddlesticks's passive automatically reduces nearby enemies' MR by 10. However, that doesn't mean I go completely AP. I get some MP with the marks, and then some mana regeneration with the seals. I then use glyhps and quints for AP.
If you don't want to use this rune setup then I urge you to get AP or MP more so than anything else. They will allow you to rape early game especially if you are facing squishies
The key to fiddle's ult is timing. I can't count how many times I barely missed an opportunity because I was slightly late. You also have to be smart, you can't assume someone's in a bush, you'd better KNOW they're in the bush.
If your whole team is pushing a lane (you should have hextech by this time), then when you reach the turret and have minions on the way, even if all of the other team is on the turret, ult in on them. If you team is good, they will follow you in, because you'll be tanking the turret hits if the other team stays, and because of your spell vamp, you should be regaining health fairly quickly. If your team is a bunch of chickens and stay back, well, may god have mercy on your soul.