#2 Sentinels
The Sentinels are giant three-story-high robots created by the U.S. government to detect and capture mutants. There are certainly other, and better built robots in the Marvel universe, but this is the only one that is created by the government on a mass scale, and always with the same purpose. They all seem armed with energy blast weapons, the ability to fly, and their mutant detection devices.

In the famous Days of Future Past storyline, the Omega Sentinels have complete control of the United States, hunting and killing mutants with ease (placing many of them in internment camps).

While the Sentinels are a technological marvel for the United States, one has to wonder why they've never dominated in present day comic story lines. They're always seen as a huge threat, but they're also always defeated by the X-Men, who aren't as powerful as even one Hulk or Thor. Of course, there's always Nimrod or Master Mold, but those are unique Sentinel threats.

The Sentinels actually reminds me of the Daleks on Dr. Who -- powerful technological weapons that are a huge threat when facing just one, but simply a minor annoyance when facing a hundred of them.