ok, that's enough bitching. let's put the facts together.
-is the guard the largest military in the galaxy?
No, the Tyranids are/is. Or the orks, if you count them as one force. humanity actually outnumbers both, but not all humans are in the guard.
-Is the imperium the most advanced faction?
Fuck no. the necrons are the most advanced, period, the the dark eldar and then the eldar (the dark eldar are most likely slightly more advanced, because they had te opportunity to develop since the fall, and had the necessity since they don't use psychic powers anymore). However, while the imperium lacks the eldar or necron supertech, and don't understand most of their own high tech stuff, the tech that they do have tends to be reliable and effective. Few can argue that the Leman Russ or the baneblade are bad tanks, for example, and bolters or meltas are pretty good weapons. the relative weakness of lasguns and flak armour comes from the fact that the imperium has to deliver millions of those to their troops every day, so something more complicated would be impossible.
-is the imperium the most powerful faction? very likely*. They are more unified then the orks, and more technologically advanced then either the orks or the tyranids. their numerical advantage makes the tau or eldar not even competitors. Dark eldar are swift, annoying, and eerily effective at what they do, but aren't a military powerhouse (because they don't need to be). Chaos can be a problem, but relies on defectors and stolen weapons, and is way to eager to fight amongst themselves to threaten the dominance of the imperium. Plus, theeir strongest forces are led by abbadon, which is a loss by default. The tyranids are the only serious threat for the imperium, even as it's sole focus, but the forces that currently are in the galaxy can be beaten, the same as the necrons.
*this is based on the current number of tyranids in the galaxy, and the fact that the necrons decided to screw the C'tan and balkanise.