ShatteredSummers Keldeo's use of Scald which is a no opportunity cost move is found on all of its sets and under the support characteristic in terms of viability the burn chance it has can pretty much make or break games. You can argue that this isn't something exclusive to other Water type users however now we talk about its offensive capabilities such as Sub CM, Scarf, Taunt Life Orb, Specs with its ever changing moveset to accommodate meta trends (think HP Electric for Gyarados, HP Bug for Celebi and Starmie, Icy Wind when the Latis were the primary checks to Keldeo). Being SR resistant is a big factor as it can come in more often than a majority of Pokemon and an offensive behemoth nonetheless, something that holds back such threats like the Zards and Pinsir. Most of your checks and counters you named are simply under the premise that on a good day you're staring at Keldeo 1v1 with nothing prior having occurred yet, which is unrealistic when we take into account factors such as the cores associated with Keldeo, the realistic scenarios that will arise based on team compositions, and the tools that Keldeo and said cores have at their disposal.
Also reiterating Celticprides 2nd point and I brought this up awhile back when ben gay asked me about its ranking at the time (this was before I was on ranking team btw), if you don't have a Keldeo answer or a way to handle it your team is simply not prepared, plain and simple. With all this being said this is why it's S rank. Isn't really a simple answer but you didn't ask a simple question so I guess it's a good read for you and others :p
Also reiterating Celticprides 2nd point and I brought this up awhile back when ben gay asked me about its ranking at the time (this was before I was on ranking team btw), if you don't have a Keldeo answer or a way to handle it your team is simply not prepared, plain and simple. With all this being said this is why it's S rank. Isn't really a simple answer but you didn't ask a simple question so I guess it's a good read for you and others :p