Being my first real lesson, I want this to be something everyone can use and appreciate, so today's lesson is about the essential Side Deck. (This will be based on April 2015 banlist).

Here are some Side Deck cards that are commonly used:
Flying "C"--Great for combating a deck that makes quick and many XYZs.
Spell Shattering Arrow--Mainly used against Qliphort, but also has uses against decks like Noble Knights.
Twister--This card is another MST; it's used to destroy cards like Vanity's Emptiness, Skill Drain, etc...
Imperial Iron Wall--The best card to use against Ritual Beasts. It also has uses against most other tier 1-3 decks.
Shadow-Imprisoning Mirror--Good against Burning Abyss and Shaddoll, but will see little use with the new banlist.
Vanity's Emptiness--Incredibly strong card and with it being limited to 1, it could be used in the Side Deck instead of the Main Deck.
Mind Crush--Arguably the most important Side Deck card in today's game. The majority of decks will add cards to their hand, making this card extremely vital.
Maxx "C"--This card is seeing more use as time passes. With Emptiness at 1, this card can substitute it; instead of preventing the Special Summon, why not plus off of it?
Light-Imprisoning Mirror-Mainly used against Tellarknights, but can be useful against other rogue decks as well.
Degenerate Circuit--A great card to use against Yosenju. Will most likely not see any other uses.
Effect Veiler--This card sees the Side Deck in some formats, and others it sits on the sidelines. It's worth considering running this format to combat Yosenju, Nekroz, etc...
Fairy Wind--Another option to destroying face-up cards. If your deck does not run any face-up Spells or Traps, this should be used over anything else that's similar.
Shared Ride-Normally only used in Nekroz match-ups (especially in the mirror), but can see other uses as well.
Artifact Lancea (thanks to Nightbot for this suggestion)--Pretty good card to use against decks that focus on banishing their cards, if your deck does as well.

Side decking is crucial in matches, and having a well-developed, diverse Side Deck will put you in a better position. If you have any other suggestions than the ones listed above, please comment!

I feel as if Anti-spell Fragrance should be on this list to vs EVERY pendulum deck.

arrow is not very effective vs noble knights as most of the equips just reequip themselves when popped

Yes, but that interrupts their strategy as their main cards destroy their equips as well.

NK being relevant tho. Anyways, Mistake? Is really good against Nekroz. Effect Vieler is also okay in side deck. I personally like Mask of Restrict works well against Nekroz and Qliphorts.

Yeah mistake is good, black horn of heaven, mirror of the ice barrier, rivalry of warlords, gozen match, chain dissapearence for shaddols, night beam, noble man of cross out