Camerupt - Worst case scenario (besides HP Water): Modest Life Orb +1 Raikou's Shadow Ball does 81.49% - 96.09%. Without a boost (meaning it predicted Camerupt's entrance) the same Raikou does 54.45% - 64.06%.
Meanwhile, max Attack Camerupt has a clean KO on 68 HP/0 Def Raikou with Earthquake.
If Camerupt invests in max HP, it takes 44.48% - 52.33%.
Nidoqueen - Takes 51.04% - 60.16% from a +1 Max SpA Modest Life Orb Raikou. (Max HP/no SDef) 3HKO even with Stealth Rock down. And then it deals 69.23% - 81.66% back with Earthquake (min Atk).
Cradily - Again from +1 Modest Life Orb Raikou. Shadow Ball deals 35.83% - 42.25% to 252 HP/96 SDef Cradily...without Sandstorm. Earthquake deals 45.56% - 53.85% back. Cradily has Recover so it wins.
Muk - +1 Modest Life Orb Thunderbolt: 56.52% - 66.67% to 252 HP/176 SDef Careful. Poison Jab deals 33.73% - 39.94%. Muk has Explosion, I guess.
Umbreon - Uh, I'm not sure about this one considering it has similar stats as Muk, only worse attack.
Lanturn: +1 Shadow Ball 2HKOs the Standard.
Keep in mind that only 8.2% of last month's Raikous were Modest, and less than 8.3% held Life Orb. (In January, 15.8% held Life Orb and 8% were Modest.) So some of the Pokemon above who fell to my extreme example can still work against some Raikou.
For example, take Lanturn vs the Standard: A +6 Timid Max SpA does 28.00% - 33.18% to a Calm 252 SDef/136 HP Lanturn with HP Ice, so it needs a crit or some other attack.
SDS's set falls to Toxic. Raikou needs 5 CM's to 2HKO Lanturn with Shadow Ball. By the time it attacks, Toxic will have sapped 13.75/16 of Raikou's health (85%), assuming Toxic does not miss and not counting the damage Lanturn has inflicted on Raikou. On the next turn, Raikou will KO Lanturn but will then die.
Of course, Camerupt fails as a counter if Raikou has HP Water, Nidoqueen does worse if it has HP Ice, HP Fighting wrecks Cradily, etc. But Raikou can't have all of those at once.
But anyway, even the most extreme Raikou will have more counters than just Regice and Chansey.