Fiddlesticks has been my main ever since level 12, and by now, I'd like to think of myself as 'experienced'. I've played all the usual methods of him, purchased two of his skins, and made my own item build. I've encountered champions strong and weak against him.

Play Fiddlesticks either mid, or jungle. Nothing else. This build will cover nearly everything I know about him, such as his skill methods and his playstyles and idt'll work for both junglers and laners.

GL HF.

P.S. What makes this build so much better than everyone else's you ask?

Well : If you go around to the other builds, compared to the level 18 stats of mine, they're inferior.

Oh, and mine has near proper English use. :)

Pros :
Huge AP output
Nice CC moves (3 second fear for example)
Near infinite health sustain (excluding the mana costs)
Viable for Ranked Play
hot dance.

Cons :
Mana hungry until about level 7
Terrible against all CC (it cancels out his drain and ulti)
Can't carry a game as well as other APs such as Ahri

The runes for Fiddlesticks don't really make too much of a difference, but here's what I've chosen.

Flat Mana Regen per 5 secs Seals - because the early game mana regen will help greatly if you use your abilites a bit more than you should

Flat Magic Penetration Marks - because PENETRATION IS WONDERFUL

Flat Ability Power Quints - for of the tiny extra power

Flat Cooldown Reduction Glyphs - because Fiddlesticks's spells do have relatively high cooldowns, and lowering them can let you cast one after the other with no wait time

The masteries I have here are near necessary for Fiddlesticks. I've poured all the points into the AP sides of the Offense category, and used the rest simply to reach the extended buff time mastery. The AP should be self-explanatory, and as I mention/ed somewhere in this build, blue buff will work magic on a Fiddlesticks.

After going through other people's builds and experimenting with my own, I've found this to give the most AP to health ratio, with 20% extra spell vamp. This'll boost his drain back to when it was before nerf, and it'll also let his E and R to heal him. I used the Rylai's for the slow over the Rod of Ages's extra health and mana.

Simple : Max your W and R ASAP, and personally I'd max E over Q first, but if you want more silence time (3 secs), max Q first.

When using your skills, use Crowstorm (R), Dark Wind (E), Terrorize (Q), Drain (W) and Ignite (D/F)

For jungling, everything is exactly the same, but with smite.
The standard route for Fiddlesticks (with leash) is :
Blue Buff > Wolves > Wraiths > Golems > Red Buff

You don't have to go in that order, but make sure to get blue buff first, or else you'll run out of mana halfway in.

When laning, it's a must to have flash and ignite for Fiddlesticks.
Flash lets you ulti over a wall and tower dive, then flash in deeper, or flash out. Ignite will prevent so many FML moments when your drain stops when they have 12 health left. Also, YOU CAN USE IGNITE WHILE YOU DRAIN.

When jungling, take Flash and Smite, for the hopefully obvious reasons.