Oda has stated that bounties are based up threat level to the government. Bounties are how much an enemy is worth to the government captured (up to 30% is removed when the bounty is killed). Leaders usually qualify for higher bounties. Only those opposed to the government have them, so marines will not be given any. Since more stronger pirates pose larger threats, usually bounty is a good rough indicator of fighting ability.
Chopper: 50 (not accurate)
Chikara(力), physical strength:
Chikara is used in the red book to indicate strengths of the Straw Hat crew. It can be literally translated as "physical force". The kanji's original representation is an arm, and means only physical strength when the kanji is used alone in referrence to a person. It is used alongside other dubious attributes. This measurement has only been used in the databook.
Dexterity is used in the red book to indicate precise manipulation (tool usage, etc.). Usopp scores highest in this category, as his high dexterity makes him an excellent sniper and tool maker. Zoro is not at the highest value, indicating a weakness in his swordsmanship (made up with power instead). This measurement has only been used in the databook.
Douriki is tested using teawase (a game/contest). Members of Cipher Pol 9 are the only people tested so far. The contest only tests physical strength, so it is similar to chikara. It does not take into account the extent of DF abilities. It is unknown whether this measurement will be used by Oda later in the manga.
Normal marine: 10
Tsuyoi (強), powerful, true strength:
This is used to declare a "powerful" person, equivalent to fighting ability. The word can also be used to declare a greater/stronger person. When Cobi is defeated by Luffy, he calls Luffy powerful (強). The kanji for tsuyoi is also used in other shonen manga, such as Dragonball Z.
Whitebeard is known as the strongest (強) man in the world.