The best around is "Great War Spearhead":
http://www.greatwarspearhead.comAnother excellent option is "Trenchline" the WW1 expansion for Panzer Korps:
http://www.panzerkorps.com/index.html(Hoplite really need to sort out a decent web presence but are very helpful answering queries and have good iO groups for rule discussions)
Spearhead is more detailed and gives greater granularity to combat and support units and consequently is slower to play. Individual units act each turn in accordance with various levels of order constraints.
Trenchline (as with all the Panzer Korps range) amalgamates point and support units into a discrete entity which is the game element so is simpler and plays more quickly.
The Panzer Korps system is more akin to a 3D boardgame. You will need to get your head around the abstraction that all the units associated with the game element are acting even though they are not individually ordered. It is a very different way of figure gaming.
Both will give good games and can actually be played with the same figure basing, so the choice is how many figures do you want on the table and how much time do you want to allow for each game? As your force sizes increase or if you have time pressures Trenchline becomes the better option.
Both these systems are parts of a WW1-WW2-Modern trilogy of rules that share common mechanics and concepts so if one does work for your WW1 games you can move to later eras very easily.