It was a great shame that warhammer historical was folded by GW last year as it meant no further resource books for their mighty Great War rules.
The rule set was unashamedly based on warhammer 40,000 and is a fantastic game that captures some of the most deadly aspects of the WW1. Specifically machine guns rule the battlefield and are appropriately limited by the army lists in the first book and the follow up Over the Top.
My army of choice is the French and in particular early war, as nothing says 20th century military genius like taking to the battlefield in red pantaloons and a blue jacket with tails.
The French recently had a game against some beautifully painted Chinese as counts as British. A mighty victory was had for the French and for a full breakdown check out: http://www.westgamer.com/forum/viewtopic.php?f=84&t=27002
Most of my miniatures are from Renegade miniatures who unfortunately are in a bit of a hiatus at the moment. I also have some from Great War Miniatures.
We tend to use the late war army lists from the first book and occasionally my opponents let me use some French cavalry who are not an option in the late war French list reflecting the reality that many soldiers and their mounts took the WW1 battlefields thinking their grand cavalry charges from the napoleonic era could be replicated. Sadly they were wrong.
My army lists tend to include one company of infantry and a company of veterans. For the veterans I use the Renegade miniatures Marines.