The different neutral camps pose different levels of strength, but what are the actual numerical strengths of these camps? I am under the impression that the mercenary camp is the weakest and the boss camp the strongest, but I have nothing to support that.
The neutral camps do NOT pose different levels of strength. They are more or less useful depending on what you need them to do. There is no such thing as "weakest" or "strongest" - they are better per situation.
Although Boss camps have the highest HP, they are not necessarily the best for taking down forts in all situations.
More information about Camps can be found below.
Siege giants are large, destructive mercenaries with a ranged attack that lets them easily take down enemy forts and towns. They're not as threatening up close, however, which makes them the easiest camp to take. Siege giants come in groups of 2 identical units, with a ranged attack that deals more damage to structures.
Knights are tanky units that are excellent at fortifying creep waves. Though they're individually not as powerful, their numbers and bonuses make them a strong addition to any push. Knights come in groups of 3 knights and a mage that provides additional buffs to nearby units.
The boss exists on some maps as a mercenary camp. Grave golems are large, tanky, high damage units that can easily push through enemy forts. Bosses ignore enemy creeps and go right after forts. This can be a good or bad thing depending on whether or not you want them to siege forts without your team. Bosses have a short range, area of effect entangle as well as a large area of effect slam attack that stuns all nearby enemies.
NOTE There are differences PER MAP based on these camps. "Boss" may be equal to "Golem"...but there are essentially 3 different kinds of neutral camps you'll come across.