Nobody really knows how power is calculated and it does not follow "best weapon has best power". It is supposed to be an overall stat for thr weapon that considers rarity and damage, but also all the other stats in the weapon. However, it is not the case as for example an uncommon, level 1 SMAW troop rocket launcher has higher power than a 5 star, level 50 rare 7.62 or an epic level 30 RKT120, and the most basic rocket launcher RKT40 has the highest power of all weapons, above any legendary Hellfire missile, something you would not expect from a less rare, more basic weapon available in lower ranks. Or a level 1 rare marksman rifle has better power than a level 40 epic I37 shotgun.
Focus on damage and also overheat/cooldown for ACs, blast radius and ammo and reload time for for cannons and rockets, and ammo and lock time for missiles and gunship sniper.
In general, the higher the damage, the better, but for ACs, the highest damage is done by the 5.56, which overheats quick. Some of us prefer the lower damage of the 7.62 that overheats after a much longer boost.