I take Flash because it is very useful for getting away and closer to the enemy. You can crowstorm in then flashto get close, then fear then drain and then you can put an exhaust on them while they are feared so they stay within your crowstorm. I found exhaust is better than ignite while being fiddlesticks because they always run out of range. I managed to get more kills with exhaust that ignite. Plus my masteries buff exhaust and reduce the cooldown time of my flash.

In any fight your main weapon is drain you want to make this as strong as possible to overcome the enemys lifesteal or damage that they put on you. So that is why you want to stack items with lots of ability power. Rylai crystal scepter is excellent for fiddle as it slows enemys movement speed and provides some health for fiddle. Will of the ancients makes all of your abilities restore health so to increase the amount healed.

Since drain is the main weapon of Fiddlesticks you want the cooldown to be as low as possible so he can keep sustain himself and not leave himself defenseless after an ability is used.