Well, after a long sabbatical, I figure I’d come back with a rather difficult question, what does make a top tier deck?
To start, a really good boss monster. This might sound obvious, but without at least one clear boss monster, the deck can’t quite work as well, hence certain decks never quite making top tier. For example, my Fabled deck. Although it can synchro pretty briskly, Fabled Leviathan is no way as good as something like Abyssgaios or Grapha. A good boss monster is essential, as it gives the deck an objective; to summon it. Now, a lot of decks move beyond their boss monster, which is good, a little originality never hurt the game, but the boss gives the deck something to work with in the first couple of months of its life, thus allowing it to become a thing before the deck evolves into whatever it becomes.
Secondly, the deck must be able to search anything, quickly and reliably. For example, Wind-Ups have both Factory and Magician (even if you can only have one), which allow you to get anything you need. This searching allows the player to work around drawing, which is notoriously unreliable (for example, when you desperately need a monster but cannot draw one), and also to play any strategy they need whenever they need it. A few decks work around this by drawing relentlessly, for example Exoida decks or certain Six Samurai builds.
Thirdly, the deck cannot see the graveyard as a death for their cards. Whilst it is a common move to use the graveyard to support strategies (for example, Frogarchs or Chaos Dragons), a powerful deck will not see cards going to the graveyard as a problem, either because their strategies cannot be stopped simply by loosing just one monster, or because that monster can simply come back or be used for something else (for example, with Elemental Dragons).
Finally, the deck must have more than enough space for staples like Monster Reborn, Heavy Storm or Mystical Space Typhoon, simply because (as you probably well know) these cards are just too good to miss.
If you have any thoughts, feel free to leave them in the comments.