I am not the most experienced player here(not by a really really really long shot), but I guess I can give my two cents on S-rank Zacian.
As mentioned by others here, Zacian-Crowned is fearsome to face - even in the age of restricted pokemon, it stands out for really hitting like a truck. And while its inability to Dynamax may seem like a hindrance, it may actually be beneficial for Zacian's teammates, because now they have much more opportunities to max. This in turn allows for teams to exert more offensive pressure.
While most teams have found strategies to counter Zacian-Crowned effectively so it doesn't completely destroy Kyurem-W, Caly-I, etc., Zacian teams have also found strategies when sun comes up, or when the Steel-Psychics are on the field. Zacian's incredible speed tier and massive Attack stat also allow for it to put in a lot of work with minimal effort put in supporting it, giving more freedom in the teambuilder, which in turn means people can use a potential second offensive powerhouse that prefers to Dynamax to reach its full potential. The sheer power and versatility that Zacian offers has made it a cornerstone of the meta, to the point where the most successful teams are either Zacian teams or teams that can stop Zacian teams reliably.
For example, in the Players Cup III NA Top Cut, most teams were either running Zacian or sun(which is one of the best counters to Zacian currently in the meta, which definitely made it more popular in S8). Heck, when Wolfey was challenged to build a team with Stonjourner, the restricted he used was... you guessed it... Zacian.
The way Zacian has changed the meta in S8 is very hard to overstate, from the beginning of S8 to the end, and as such, one can argue that Zacian is one of the best pokemon in the meta right now.
When looking back at the requirement for S, it says "reserved for the metagame's best pokemon". By that logic, I am arguing that Zacian should be considered for either A+, S- or S tier.
As mentioned by others here, Zacian-Crowned is fearsome to face - even in the age of restricted pokemon, it stands out for really hitting like a truck. And while its inability to Dynamax may seem like a hindrance, it may actually be beneficial for Zacian's teammates, because now they have much more opportunities to max. This in turn allows for teams to exert more offensive pressure.
While most teams have found strategies to counter Zacian-Crowned effectively so it doesn't completely destroy Kyurem-W, Caly-I, etc., Zacian teams have also found strategies when sun comes up, or when the Steel-Psychics are on the field. Zacian's incredible speed tier and massive Attack stat also allow for it to put in a lot of work with minimal effort put in supporting it, giving more freedom in the teambuilder, which in turn means people can use a potential second offensive powerhouse that prefers to Dynamax to reach its full potential. The sheer power and versatility that Zacian offers has made it a cornerstone of the meta, to the point where the most successful teams are either Zacian teams or teams that can stop Zacian teams reliably.
For example, in the Players Cup III NA Top Cut, most teams were either running Zacian or sun(which is one of the best counters to Zacian currently in the meta, which definitely made it more popular in S8). Heck, when Wolfey was challenged to build a team with Stonjourner, the restricted he used was... you guessed it... Zacian.
The way Zacian has changed the meta in S8 is very hard to overstate, from the beginning of S8 to the end, and as such, one can argue that Zacian is one of the best pokemon in the meta right now.
When looking back at the requirement for S, it says "reserved for the metagame's best pokemon". By that logic, I am arguing that Zacian should be considered for either A+, S- or S tier.