Q - Terrify
Fiddle's Q has a passive and active, making it useful in multiple situations. I'll talk about both here:
Terrify's Passive allows fiddle to dread all enemies when attacking after not being seen and out of combat for a set duration of time, and his next attacking ability fears the enemy for a max of 2 seconds when the ability is maxed out. The key thing here is that he can dread/fear multiple enemies, meaning that in large scale team fights, you can appear out of nowhere with an ult or W and fear the entire team, allowing your team to come in and dish out INSANE damage, tipping the scales of the game. If your communication is good and you land your abilities, you can win your team a fight instantly.
His Q is also amazing for clearing multiple-enemy camps such as Reports, as his fear will allow him to clear it much faster without taking much damage.
Fiddlesticks "Shrieks" at an enemy dealing up to 10% + 2% per 100 AP of the targets HP in MD as well as fearing them. Enemies are only able to be feared every few seconds, and instead does damage to an enemy if that said enemy has been recently feared. The min-damage and active damage thresholds are doubled if recently feared and enemies that are struck with fear are slowed by 90%. This is really good for dealing additional damage and finishing off enemies that are low after a W E and even an R.
W - Bountiful Harvest
Fiddle's W is probably my favorite part of their kit. Fiddlesticks binds himself to ALLLLL nearby enemies, standing still and dealing progressive damage to the target(s), siphoning their souls. Dealing up to 42.5 + 8.25% of AP in MD when maxed every .25 seconds and a final up to 42.5 + 8.25% of AP + 16% of the targets missing HP as a sort of executing damage. Fiddle then heals himself up to 50% of the pre-mitigation DMG dealt, reduced to 15% against minions. If targets break their binds by moving out of his W's range, the ability ends instantly. Fiddle's W is so good, as it's camp clear is insane against multiple-enemy camps, but is also good, as stated previously, in large scale team fights, as is able to W onto all nearby enemies and do damage while healing as his team is also dealing damage. Using this after using your R is super good as your damage from the R will combine with your W's damage and deal huge amounts of overall damage.