Every time a new standard rotation comes about, I go through every card and create a spreadsheet of deck ideas. This is a lengthy process but I don't mind because I enjoy it. One thing which makes this process much easier is what I call 'filters'. That is, cards which are very powerful relative to the rest of standard. Those few powerful cards will be very popular, so much in fact that any card which matches up against them poorly, is filtered out of the format, or at least into people's sideboards.
You've probably heard someone say "'X' warps the format". That card, 'X' is probably a filter. Examples of filters in the past include: Flametongue Kavu, Jace, the Mind Sculptor, Bitterblossom, Siege Rhino, etc.
The current standard format is Dragons of Tarkir through to Shadows over Innistrad. I'm going list the cards which I believe are filters and explain why.
Fleecemane Lion and Strangleroot Geist are two which come to mind (dunno why those two, Fleecy for sure came to mind because of my longing for Nick Watson). When Fleecemane Lion and Strangleroot Geist attack, you can at least attack them back with your two-power two drop. Sylvan Advocate though, blanks any two-drop without evasion or three power. Goodbye Scourge Wolf, goodbye Knight of the White Orchid (Knight of the Quite Awkward), etc.
"But Luke, Knight of the White Orchard does see play". Yep it does, but only because the deck it's in has plenty of cards to give that extra power to overcome Sylvan Advocate. I'd rather not rely on drawing my combo to overcome a staple of the format thank you. I want all my cards to match up decently by themselves.
Here's a list of in-my-opinion the best two-drop creatures in Standard:
Ayli, Eternal Pilgrim
Forerunner of Slaughter
Avatar of the Resolute
Anafenza, Kin-Tree Spirit
Sylvan Advocate is tier 1 because it rumbles with everything but Ayli early on and later in the game it's a huge 4/5 vigilance.
Ayli is held back by her legendary status and restrictive mana cost.
Stormchaser Mage and Forerunner of Slaughter are underrepresented at the moment, particularly Stormchaser Mage as he survives a friendly Radiant Flames.
Kessig Forgemaster is, I believe the most underrated card in standard.
Avatar of the Resolute and Anafenza are too restrictive on mana-bases to be worth it.
Bearer of Silence
The mana cost of a filter card determines which cards it filters out. For example, if a filter card costs five mana to cast, it isn't going to filter out any one-cost 2/1's because they'll get in enough damage to be good before the five-cost gets cast.
The mana cost factor is more relevant for removal, any creature which doesn't give you any value if removed, e.g. Polukranos, World Eater, shouldn't cost more than two mana more than any of the prevalent removal spells of the format. That's my rule at least. Back in Theros, Hero's Downfall was the premier removal spell (filter) and Polukranos passed the two-mana test.
I should put a caveat in here; sometimes a card is SO POWERFUL that even though there is the potential for you to get blown out, if they don't have the removal spell, you pretty much win the game. Baneslayer Angel was one such card. Doom Blade and Terminate were in her format, but sometimes you don't draw the removal and you lose. Cards like that are bad for magic.
Any dude which doesn't provide any 'enters the battlefield' trigger value and costs more than three, get's rekt by Reflector Mage. There's a reason why you don't see Woodland Wanderer wandering around or Endbringer bringing ends.
Kalitas, Traitor of Ghet is kind of an exception. Part of the reason he still sees play despite getting rekt by Reflector Mage (Ref-rekt-or Mage) is that if you have a removal spell like Fiery Impulse on the same turn you cast Kalitas, you'll end up with a zombie. The other part of the reason he still sees play is how much of a Baneslayer he is.
Reflector Mage's 2/3 body matches Sylvan Advocate's, doubling the importance of choosing creatures which fight a 2/3 well.
An ironic thing about cards which act as filters, is that they weaken themselves as the format develops: players adapt and stop playing cards which are poor against filters. If you're in tune with when your local metagame reaches this point, you might be able to get away with leaving your poorly timed filters in the sideboard.
Jace, Unraveler of Secrets is a card which I rate highly but his -2 ability isn't very effective in a format where everyone has already adjusted to Reflector Mage. It's like Jace, Unraveller of Secrets was Starbucks and Reflector Mage was Melbourne's well established coffee cafe's...there's a filter coffee joke in there somewhere.
Chandra, Fire of Kaladesh got no play? I attribute that to Fiery Impulse. Similar to how Anax and Cymede never got any play because Wild Slash was popular.
Fiery Impulse's spell mastery is too hard to turn on by turn three, so X/3's on turn three are safe enough from Fiery Impulse.
Kozilek's Return compliments Fiery Impulse's warding effect on three-cost X/2's. To a lesser degree, so does Flaying Tendrils, Seismic Rupture and Gideon's knight tokens.
Those are the only filters which come to mind at this stage. If you're brewing a deck for your next tournament, keep these four cards in mind and you should be ok.