The highest CP in Pokemon Go is 6672, which can be achieved by turning Groudon and Kyogre into Primal Groudon and Primal Kyogre through Primal Reversion. However, Primal Reversions are temporary in the game, and when removed, the highest attainable CP is 5010, which belongs to Slaking. If we take mega-evolutions out of the picture, Slaking has the highest maximum CP in Pokemon Go. Here are the top 10 Pokemon with the highest CP in Pokemon Go:

• Mega Latios – 5661 CP
• Mega Salamence – 5688 CP
• Mega Latias – 5428 CP
• Mega Gyarados – 5332 CP
• Mega Charizard Y – 5037 CP
• Slaking – 5010 CP
• Regigigas – 4913 CP
• Mega Gengar – 4902 CP
• Mega Ampharos – 4799 CP
• Mewtwo – 4724 CP

Its worth noting that mega-evolutions are temporary and require the use of "Mega Energy" on the standard Pokemon, so their high CP values are not always available.