I would like to remind everyone that checks =/= counters. Things like Dugtrio, Breloom and Suicune are not counters (and cap10 is also more than capable of beating them one on one).
First of all, let's not forget that CAP10 has a great sweeping potential. Although its attacking stats are average, Magic Guard and Garchomp-esque bulk more than make up for it (4/0 CAP10 has more physical bulk than a 4/0 Garchomp). Here are some calcs from a Life Orbed, max SA, Timid CAP10
Ice Beam vs 4/0 Breloom: 115.7% min
If anything, CAP10 checks Breloom, lol.
Thunderbolt vs 252/0 Suicune: 65.8% - 77.7%
That puts an awful lot of dent in Suicune IMO. Even if it swaps into a +1 Cune it can chop off a nice 46% and if there is sandstorm, Suicune can be in a lot of trouble. Also, the possibility of Charge Beam/Psych Up in its movepool can help it bypass the CM boosts
Surf vs 252/0 Swampert: 40%-48%
Ice Beam vs 252/0 Celebi: 49.5% - 58.1%
Careful here...., though that's not to say that Celebi can't be customized to beat CAP10, given the somewhat low damage without investment in SDef. Be careful of entry hazards
(This also means Surf/Thunderbolt does ~37% min to 252/0 Jirachi)
Ice Beam vs 4/0 Roserade: 73.5%-86%
That said, if we don't give CAP10 Recover/Volt Tackle/Superpower (at least together with Magic Guard), I can see Blissey and Snorlax being the closest thing to solid counters. Blissey can shrug off all of the Sspecial attacks and slowly whittle it down with Seismic Toss, and Snorlax can do the same, except by utilizing Earthquake to dispose of CAP10 faster. Celebi is a possibility too, but it'd need Leaf Storm to do so, and prays that CAP10 will not have stuff like Fire Blast or Bug Buzz in its movepool.
As CAP10 is supposed to counter a wide range of Pokemon (though one at a time) it's really hard to say what it won't ever deal with.
First of all, let's not forget that CAP10 has a great sweeping potential. Although its attacking stats are average, Magic Guard and Garchomp-esque bulk more than make up for it (4/0 CAP10 has more physical bulk than a 4/0 Garchomp). Here are some calcs from a Life Orbed, max SA, Timid CAP10
Ice Beam vs 4/0 Breloom: 115.7% min
If anything, CAP10 checks Breloom, lol.
Thunderbolt vs 252/0 Suicune: 65.8% - 77.7%
That puts an awful lot of dent in Suicune IMO. Even if it swaps into a +1 Cune it can chop off a nice 46% and if there is sandstorm, Suicune can be in a lot of trouble. Also, the possibility of Charge Beam/Psych Up in its movepool can help it bypass the CM boosts
Surf vs 252/0 Swampert: 40%-48%
Ice Beam vs 252/0 Celebi: 49.5% - 58.1%
Careful here...., though that's not to say that Celebi can't be customized to beat CAP10, given the somewhat low damage without investment in SDef. Be careful of entry hazards
(This also means Surf/Thunderbolt does ~37% min to 252/0 Jirachi)
Ice Beam vs 4/0 Roserade: 73.5%-86%
That said, if we don't give CAP10 Recover/Volt Tackle/Superpower (at least together with Magic Guard), I can see Blissey and Snorlax being the closest thing to solid counters. Blissey can shrug off all of the Sspecial attacks and slowly whittle it down with Seismic Toss, and Snorlax can do the same, except by utilizing Earthquake to dispose of CAP10 faster. Celebi is a possibility too, but it'd need Leaf Storm to do so, and prays that CAP10 will not have stuff like Fire Blast or Bug Buzz in its movepool.
As CAP10 is supposed to counter a wide range of Pokemon (though one at a time) it's really hard to say what it won't ever deal with.