Fiddlesticks has a nice set of skills - a decent length disable, a bouncing silence, and the potent life drain channel. They're all very nice to have... which has caused me great trouble in figuring out how to prioritize the skills I'm learning. I tend to max Drain first, but continually flip-flop on Dark Wind and Terrify. You typically have a goal when you decide on a general skill order (best damage improvement, maximizing disable amount, minimizing mana-to-utility ratio, etc.), but the synergy and utility of Fiddlesticks' arsenal has left me rather confused on what goals to focus with. As a result, I never seem to achieve effectiveness in combat.
What are the focus points of building Fiddlesticks in terms of skill learning? Are there certain factors (item build, enemy setup) which would imply a specific skill order, and what factors of those make such implications?