Looks like the discussion has moved on slightly, but I've been thinking about this.
As Aldaron mentioned, there rarely are '100% counters' to anything anymore. There is no 'Garchomp counter'. There is no 'Lucario counter'. Most of these sets are entirely counterable. For example, for Lucario, Weezing counters any physical based variant, and Tentacruel beats Specs users.
However, when Lucario comes out, you have no idea of the set. Say I have both Weezing and Tentacruel; I could send either of them in, but despite countering their respective sets, it's certainly got an element of risk to it. The whole 'SD Yache Chomp' thing seems to be the only really common exception to this.
As far as I'm concerned, much of the ambiguity in this thread is caused by differing definitions of the term 'counter'. There are obviously different types of counter.
1) 'Definitive' counters; something that can come in on any standard variation of the Poke with relative ease (As in, something that has some kind of use bar getting past one specific counter, so not Hp Ice Heracross or Natural Gift Skarmory) and pose some sort of threat as a result, whether it be killing it or setting up Spikes in it's face, or something else entirely. A counter cannot be a part of this category if it fits into either of the other two categories.
Examples of this would be Porygon2 coming into Gyarados, or Cresselia coming into Electivire. The former can Intimidate it back thanks to Trace, take a hit and OHKO with T-Bolt, whilst in the latter, whilst it will take a while for Cresselia to kill off Electivire, due to reliable recovery it will overcome all forms of Electivire, regardless of moveset.
2) 'Specific set' counters; something that can counter some movesets of Pokemon, but are less useful against others.
Examples of this would include Blissey countering Salamence; it counters the SpecsMence beautifully, but doesn't stand a chance against DD, CB, or mix. Hippowdon, can switch into DDTar amd CBTar with ease, but is 2HKO'd by Boah Ice Beam.
3) 'Prediction based' counters; something that can switch in on some moves of a particular set, but cannot switch in against one or more of the other moves.
Examples of this would most obviously be Choiced Pokes; Skarmory can come in on a Heracross CB Megahorn or Stone Edge, but not Close Combat as that is a potential 2HKO. Bronzong can come in on Specsmence Draco Meteor, but risks an OHKO from Specs Fire Blast. It's not solely limited to choiced Pokes though; for example, Vaporeon can come in on anything the standard Mixape carries except Close Combat, as that is a potential 2HKO. Some 'counters' in this category may fit into the second category too.
From this, it seems that as 'SD Yache Garchomp' counters go, it's both part of two and three. Cresselia takea less than 50% all the time from a +2 359 Dragon Claw, and manages to 2HKO even with Yache. So, Outrage SD Yache would be the only set which is truly 'uncounterable'. But, assuming you can predict the Outrage, and switch to your Dragon resist which every team should carry, then it's a sitting duck.
Times have moved on since GSC; you can no longer efficiently counter the entire metagame without some degree of prediction. The fact that there is nothing that can switch into SD Yache Garchomp without some element of risk doesn't make the set uncounterable; just that you will almost certainly need to predict to do so.
As Aldaron mentioned, there rarely are '100% counters' to anything anymore. There is no 'Garchomp counter'. There is no 'Lucario counter'. Most of these sets are entirely counterable. For example, for Lucario, Weezing counters any physical based variant, and Tentacruel beats Specs users.
However, when Lucario comes out, you have no idea of the set. Say I have both Weezing and Tentacruel; I could send either of them in, but despite countering their respective sets, it's certainly got an element of risk to it. The whole 'SD Yache Chomp' thing seems to be the only really common exception to this.
As far as I'm concerned, much of the ambiguity in this thread is caused by differing definitions of the term 'counter'. There are obviously different types of counter.
1) 'Definitive' counters; something that can come in on any standard variation of the Poke with relative ease (As in, something that has some kind of use bar getting past one specific counter, so not Hp Ice Heracross or Natural Gift Skarmory) and pose some sort of threat as a result, whether it be killing it or setting up Spikes in it's face, or something else entirely. A counter cannot be a part of this category if it fits into either of the other two categories.
Examples of this would be Porygon2 coming into Gyarados, or Cresselia coming into Electivire. The former can Intimidate it back thanks to Trace, take a hit and OHKO with T-Bolt, whilst in the latter, whilst it will take a while for Cresselia to kill off Electivire, due to reliable recovery it will overcome all forms of Electivire, regardless of moveset.
2) 'Specific set' counters; something that can counter some movesets of Pokemon, but are less useful against others.
Examples of this would include Blissey countering Salamence; it counters the SpecsMence beautifully, but doesn't stand a chance against DD, CB, or mix. Hippowdon, can switch into DDTar amd CBTar with ease, but is 2HKO'd by Boah Ice Beam.
3) 'Prediction based' counters; something that can switch in on some moves of a particular set, but cannot switch in against one or more of the other moves.
Examples of this would most obviously be Choiced Pokes; Skarmory can come in on a Heracross CB Megahorn or Stone Edge, but not Close Combat as that is a potential 2HKO. Bronzong can come in on Specsmence Draco Meteor, but risks an OHKO from Specs Fire Blast. It's not solely limited to choiced Pokes though; for example, Vaporeon can come in on anything the standard Mixape carries except Close Combat, as that is a potential 2HKO. Some 'counters' in this category may fit into the second category too.
From this, it seems that as 'SD Yache Garchomp' counters go, it's both part of two and three. Cresselia takea less than 50% all the time from a +2 359 Dragon Claw, and manages to 2HKO even with Yache. So, Outrage SD Yache would be the only set which is truly 'uncounterable'. But, assuming you can predict the Outrage, and switch to your Dragon resist which every team should carry, then it's a sitting duck.
Times have moved on since GSC; you can no longer efficiently counter the entire metagame without some degree of prediction. The fact that there is nothing that can switch into SD Yache Garchomp without some element of risk doesn't make the set uncounterable; just that you will almost certainly need to predict to do so.