I use a couple. Tamiya's Fine Surface Primer is probably my favorite. I use both the gray, which is a medium gray, and the lighter gray. It is very finely-grained, so it doesn't obscure finely sculpted detail, but it provides a good tooth for color coats.
I also use Rustoleum's primer, which covers well, but is a little coarser than Tamiya's primer. That may be because a painter might wind up sanding that coat, I don't know. It covers well, too.
And I use Walmart's house brand of automotive primer. Performs as well as the Rustoleum primer. I'll buy one or the other, depending on the price at our local stores. I did get a couple of bad cans, though-the nozzles and apertures at the top of the can got clogged about halfway through use. Even though I tried cleaning them, they just clogged, and I wound up losing half a can's worth of paint. That hasn't happened in a while, though.
As far as Army Painter is concerned, I haven't tried their primers, but I do use their matte black enamel as a primer on my Maschinen Krieger kits. The black enamel serves as a base for my weathering technique, which consists of using acrylics over the enamel base, and then scuffing the color coats with a green scouring pad to reveal the black underneath. That enamel is rock-hard, also covers well, and doesn't obscure fine detail.
Hope that helps!
Prost!
Brad