Does maximum Combat Power change for different Pokemon? [duplicate]

Every Pokemon has an individually assigned max CP, which is determined by the Pokemon's individual max stats (HP, attack, defense, and speed). For example, Dragonite has the highest CP of any available Pokemon (3500), but while that's the highest CP a Dragonite can get to, that doesn't mean that your Dragonite will be capable of reaching that level.

When you catch a Pokemon, it'll have bonuses to each stat (called Individual Values, or IVs) that add anywhere from 0 to 15 points to each stat. Thus a Pokemon with max IVs will have a higher CP than one with the lowest possible IVs.

The bar above the Pokemon will go up as you power up the Pokemon and raise its CP, but it actually represents the Pokemon's level (no clue why Niantic doesn't just show us the level as a number). When you fully powered up a Pokemon, the bar will be full showing the Pokemon is at the highest level possible (for your player level), and so you can have two Pokemon with full bars but different CPs.

That said, the max CP you can raise a Pokemon to is dependent on your player level. As you level up, you'll be able to level up your Pokemon further, and further raise the Pokemon's CP and stats.

And answering your question on which Pokemon has the highest CP, right now the answer is Dragonite until Mewtwo becomes available. Don't forget though that even with the highest CP Pokemon protecting a gym, someone with the right type advantages and decent dodging skills will have no problem taking your Pokemon down.

Information from thesilphroad.com