Does League of Legends currently have a Sivir problem? She's been a dominant presence across multiple continents, boasting a win rate that verges on making her ban-worthy. Yet it's hard to immediately identify why this is so.
On paper, she isn't the most intimidating AD carry. Her short auto-attack range is relatively unsafe, and though her poke is pretty good, her overall burst damage is usually outmatched by other options. Were it safety in fights, you'd usually want the long auto-attack range from Caitlyn, Jinx, Ashe or Kog'maw—with a good tank line, it's difficult to trap them with any form of crowd control. Were it raw damage, Vayne, Kalista and Corki offer execution power that leaves Sivir in the pale. Her attack steroids are nothing much to boast of either, compared to the attack speed bonuses offered by other carries.
Yet to analyze Sivir based off her combat stats alone is to miss the forest for the trees. Certain pro players might dismiss her contributions as "just a Sivir comp," but it says a lot about their own ignorance to treat it with such low priority despite its proven success.
Sivir's team contributions exist along two fronts: map control and power amplification. The first is easy enough to understand: she deletes waves by pressing two buttons. A couple Boomerang Blade and Ricochet combos later, and the enemy bot lane duo's stuck farming under the turret while she's free to roam elsewhere.
Her wave control is especially relevant in organized play, where two-on-two bot lanes aren't necessarily the guaranteed norm, nor are extended laning phases. Fast and reliable waveclear allows her team to focus on objectives early on and abuse early-game matchup advantages consistently. Either she gets an easy 2v1 lane, or she guarantees that her top laner has the better 1v1 option. In combination with the "sixth-man" pressure from the lane she's pushed up, the enemy team will find themselves regularly split along two fronts, limiting their strategic options.
If they're the sort of team that needs a long time to ramp up to their power spikes, that's a deadly sort of pressure to face. Tear of the Goddess-using champions like mid lane Kog'maw or Cassiopeia, in particular, are pressed hard to find any sort of relevance—by the time they can afford to do anything but play defensively, their team will often be down the entire front line of towers as well as multiple Dragons.
But other AD carries can do a similar job without being nearly as dangerous as Sivir. Corki's burst and poke can smash through minion waves just as well, after all, and he too is a powerful early-game pick. Unlike Sivir, however, Corki only has his own early game to rely on—he doesn't bring a whole lot to everybody else's kit.
On The Hunt might secretly be the best ultimate spell in the game. It might not do damage, nor does it offer or affect direct crowd control spells, but the powerful team-wide mobility does two extremely important things. Obviously it tips just-barely-infeasible champions into godlike status: the speed boost makes it much easier for champions like Rumble and Maokai to get into position for their spells, and makes even traditionally kite-able champions like Olaf into a real threat in the face of all of the dashes, blinks and Ghosts that litters the game. In a lot of cases, the only reason why a champion isn't "good" is because of their lack of comparative mobility, and On The Hunt is a teamwide solution to that problem.
Related to that is the true secret to Sivir's success: if her wave control can be considered a strategic seal on the enemy team, On The Hunt is an indirect form of crowd control that seals their micro-level tactical options too.
The best way to think about it is as a range extension to her allies' own crowd control effects. Maokai's Twisted Advance, Morgana's Soul Shackles, Gragas's Explosive Cask: it's that much harder to stay out of their radii when Sivir is nearby to bolster their movement speed. Even before the first spell is cast, the enemy team finds their movement options sealed off as surely as if the choke points had been blocked off with Azir's Emperor's Divide.
Coincidentally, Azir's exactly the sort of champion you want to run when facing a Sivir-reliant composition. Mass-knockback spells in general are the best tools versus On The Hunt, opening up vital space and movement options when Sivir's team decides to engage. For that, you'd also want Gragas, Janna or Tristana—Explosive Cask, Monsoon and Buster Shot respectively.
Of course, while that opens up your team's skirmish-level options, that doesn't do enough in the face of Sivir's map control—it's nice to have when her team decides to force fights instead, but stopping them from simply out-rotating you is difficult at best.
For that, you want to fight fire with fire.
Righteous Glory's one of the best options you can slap on a tank right now, thanks to the whopping 60% movement speed bonus it provides your team on activation, making it the equivalent of a mid-game On The Hunt. Area of Effect denial, like with Viktor's Gravity Field, pre-empts Sivir's movement options and denies her team critical chokepoints.
And then there's the option of being so inherently mobile that even On The Hunt can't close the gap for her team. Early-game Warrior junglers like Lee Sin and Jarvan are increasingly popular again, thanks to their huge inherent mobility and efficiency in shutting down Sivir early. But the most interesting choice I've seen so far's back home in Taipei's LoL Master Series, where mid lane maestro Westdoor's fallen back to an old favorite.
Global and pseudo-global Teleport effects are the best forms of mobility money—or RP—can buy. Shen can almost literally be in two places at once, counteracting Sivir's dominant map play, and his energy usage buffs have made the classic tank relevant again. But Twisted Fate is an even more interesting option. At first glance, his reliance on Gold Card to set up kills is a relatively poor choice versus Sivir, as she can simply Spell Shield it into oblivion (and give her extra mana for another Boomerang Blade at that). But the real threat from Twisted Fate is information.
Destiny is like having free wards up across the map. No matter what Sivir does, or how fast her team rotates, Westdoor's Twisted Fate means they should never be able to catch the enemy team by surprise. Her wave control advantages are largely mitigated if the enemy team doesn't actually have to choose between pushing a lane or fighting over Dragon, but gets to do both at the same time. Theoretically, Pantheon and Gangplank can do the same as well, thanks to their own global effects—though not Karthus, as Requiem is simply free mana for Sivir.
The big question at the moment: should Sivir be nerfed? In the face of her obvious strengths at the professional level, it's tempting to say so. She has an extremely centralizing effect in the game at the moment—it's hard to say otherwise when she isn't just in every game, but actively sought after in the first couple pick rotations.
But hold that thought. Sivir's centralization might suck for AD carries that are bored of basically playing a support role, but it's hard to say that she's bad for the game. As mentioned, she increases champion diversity for literally every other role, so even if the AD carries are bored, everybody else is having a grand old time dusting off champions that were formerly held back in a Sivir-less metagame.
What needs to be done, instead, is to decrease the current meta's reliance on On The Hunt, and Riot's already doing so. Improved gold gains with Ancient Coin makes Talisman of Ascension a much more feasible option for support players. Even better, the soft cap mechanic on movement speeds makes stacking their bonuses highly inefficient anyhow, so it's not as if Sivir teams benefit even more from having the Talisman available on their side too.
In a rare once-a-year case, I have nothing to criticize about Riot's balance plans... for now. If the lessons we've derived from Sivir's current dominance is anything to go by, the upcoming item rebalances will do a lot to improve the game's overall health.