Have you ever played against a deck and thought to your self, "If I only had 'X' card I would win?" I for one have. The reason most often is that I never though I would need such a card. Back in the day, when I got my Regional Top's, I had to side deck vs one, maybe two decks. Rogue and Anti-Meta were all but non-existent. But now, there are FAR many more decks to play and side against, and this is even more important in a format where the tier one is not one or two decks, but several.
Several decks forces the situation where players need to cut some cards from the side deck, pick more generic support, and in a worst case scenario side cards which simply act as counter-counters, as people will most often side the same cards to stop your theme. This creates an environment where good decks lose due to simple bad side deck choices or idiotic side decking in general.
Utility of the Side deck.
When you build a side deck, you must aim to cram in cards which counter the most top decks. No matter what you run, the best case situation is one in which you are in a mirror match. With that mind frame in place, your side deck must be able to handle every possible match-up. This is where a cards utility comes into play.
In your main deck you cannot play cards which hurt a decks synergy in sake of limiting a specific deck. But in a side deck, you can tarnish your decks general flow of strategy to stop your opponent from making a game winning play. Cards utility change depending on the deck the card is playing against. Utility also applies to the side deck in general, as utility also describes how well a card is against more than one match-up, or how useful the card is, although that is begging the direct definition of the word utility.
In your 15 card side deck, you must be able to counter each top deck, and be ready for the side deck. But you also need to be able to handle rogue decks. In any event, you must assume the first few rounds will be rogue decks, or not the tier one. And if we use SJC/YCS events for a guidance point for top level play, last jump had a Magical Explosion deck top, and the jump before it had a top 40 Exodia FTK/OTK deck appear, missing out due to tier breakers. Due to this, utility will be applicable even in later rounds, and potentially if you top. A side deck using efficient utility will win you more games.
Retro vs Modern Side Decking.
Some cards have become classic cards, that will always see play. In a side deck, Royal Oppression does in fact stop the most decks. Next one would side Skill Drain, as only a very select amount of decks do not exploit effect monsters, and at that, they very seldom top events. But every time a new deck arises, players dig into the common box for a tech to side for it. This is a good idea to find a useful answer to a card with a Retro option, but it is not always the best option.
The best example I can give is D.D. Crow and Crevice into a Different Dimension. D.D. Crow is the Retro option, it fades into side decks depending if the grave is useful. During Return-DAD, a late game crow could cost you the game rather than help, but in more recent formats it is increasingly potent as a card to prevent a big push. To that notion, new support does often come out to handle top decks, like Crevice. Crevice does handle more than one target, but is slower. So depending on the speed of the format, it may be more or less beneficial to side deck one or another.
Other factors come into play as to which to pick, but the general rule of thumb is to use the quickest and most live card, which goes back to utility. Crow is quicker, and you do not need to set it, making it always a live card when in hand, and is not a card which can easily be stopped. This makes it a much superior option.
High Utility in this Meta.
If I were to need to make a generic side deck, I would pick a lot of Classic/Retro cards which stop or hinder major aspects of play. But this is not my goal here, nor should it be for any player. One must side, for the sake of repetition, side cards which directly hurt the tier one. If we assume the tier one/two is composed of Frogs, Sabers, and Infernity decks, it becomes much more clear to build a side deck.
For the sake of building a side deck which is relavent for a large event, I will use Chicago as my reference point. Frogs, Gladiator Beasts, Infernity, and Sabers compose most of the Meta. And I will also factor in the premise aforementioned, of a plethora of rogue decks being run. Some cards instantly are superior options in this format, and others fit the bill ideally.
The Chicago Side.
To build a generic side deck for Chicago, it is best to address your decks best and worst match-up for the sake of space. Additionally, having a main deck to work off of greatly influences what one will side, as if I were working with a Black Wing deck, I would side Skill Drains. But, with just the decks one is playing against to work with, ideal is the best to stride for, as perfect depends on your deck solely.
The first card(s) to include would be Royal Oppressions. Two stops/hurts Glads, Infernity, well, the vast Majority of the Meta. Another solid option to counter Infernity decks is D.D. Crow. Two Crows and Two Royal Oppressions are four cards which are favorable against more than one deck. I personally would never run more than two/three cards per a specific deck in this format for the sake of space.
Next is X-Sabers. To stop them, Oppression is the only solid card. But to combat them, side two Gottoms' Emergency Call. Fight fire with fire is the best option for that match-up. For Gladiator Beasts you should run three Mirror of Oaths. Three because unlike Gottoms' Emergency Call in the Sabers match-up, it is always live. You can turn one it and use it. Additionally, Mirror of Oath is at worst a thinning card due to your plus one, meaning it will not hinder your strategy. This rounds off nine cards in the side deck.
I next address Frogs, and do so with two Chain Disappearance. It cripples all forms of frog decks. For the sake of space, I am limited to two copies, but it also hits the Infernity deck and LightSworn deck respectively, plus Quickdraw/Cat. It is situational in those match-ups but it is useful, meaning it is never a bad option. The last slot for deck specific support will be System Down/Cyber Dragon. Two copies of either to battle Machine variants. I would go with System Down as it is more generic and at worst only cripples your opponent, while Cyber Dragon takes a little more wiggling to utilize.
That is thirteen cards thus far. The last two easily go to Dust Tornado. The ability to protect big plays and support your pushes in the game is a very important trait to respect. It is also your answer for the vast majority of Anti-Meta/Rogue decks, as most Anti-Meta decks rely upon the back row and Rogue normally needs the back row for combination oriented plays.
How to Decide Between Options.
When side decking, as aforementioned, some cards work differently in your deck as others. Back to my example of cards to shut down the Infernity deck. Royal Oppression, D.D. Crow and Crevice into the Different Dimension are solid picks. But so it Consecrated Light. D.D. Crow is essentially a spell card by mechanic. To that avail, it depends on what your deck uses best. If you run a Twilight deck with teched Jinzo's and Royal Decree's, Oppression and Crevice are less than ideal.
On the same note, Twilight can remove tech to add in more options quite easily. So Consecrated Light may very well be the superior choice in such a deck. This is not the best example of how a card will or will not fair in differently in a top deck, and a Black Wing deck is a much better example, but the point is clear. The point is you must make the Meta call as to what cards do best based upon your decks specific ability to run cards. With this notion in mind, a deck with traps that do not help nor hurt the theme but needs all its traps, Crevice is superior to D.D. Crow due to space alone.
This is the side deck format for a reason. Each top deck has a clear answer or major tech to stop it. Abusing this is the difference between winning and losing. But remember utility and ability to run such cards in your deck, and winning will be less upon skill and more upon preparation. To not push your luck, come prepared.