Place 2: The Imperium of Mankind
Space Marines are among the strongest warriors of the Imperium:
Just ahead of the Necrons, the Imperium of Mankind reaches the podium. For millennia, humans have continually spread and are now the dominant species in the galaxy.
The Imperium itself is relatively young but has significantly increased its striking power since then. Alone the genetically engineered super soldiers of the Space Marines constitute a force that could easily obliterate smaller races.
Additionally, there is the Ecclesiarchy, a fanatical church with a militant order of women that is almost on par with the Space Marines. If that is not enough, the Imperial Army comes into play, which numerically outnumbers (almost) all others.
The great weakness of the Imperium is its dependence on the Emperor. If he were to lose his battle, the entire empire would be doomed – at least very likely. However, as it currently stands, the Imperium is one of the biggest players. You can read more about it in the special: Everyone fights in Warhammer 40,000 for the Imperium of Mankind – what is it actually?