Imperial Guard
The "Hammer of the Emperor", the Imperial Guard, are the main and most numerous fighting force of Mankind. Scattered across the universe, divided into countless armies, the Imperial Guard are concentrated mostly where the fighting is most fierce. With hundreds to thousands of infantry, battle tanks and thunderous artillery, the Imperial Guard crush the foes of the Imperium with brute force.
The Imperial Guard probably have more units than any other army. With regular infantry, Catachan fighters, Ogryns, Kasrkin Squads, Sentinels, Valkyries, and tons of tanks, you have so many choices if you choose to paint Imperial Guard.
Painting Imperial Guard can be pretty difficult. Besides the basic infantry, the armor on most of the units is pretty detailed and hard to paint. Also, faces are very, very difficult to paint ( at least for me - I have never painted a face that looked even close to human). And because almost all infantry units have uncovered faces, you will run into this problem a lot. In my opinion, a model with a poorly painted face makes the whole model look bad.
Overview: An Imperial Guard army has a lot of models, so you should only choose them if you really like painting. I would also recommend them to more advanced painters, as most of their units take some skill to paint correctly.