I guess you won't be surprised that you aren't the first to ask (nor the last, I bet). If we get enough input here I make this a sticky topic.
There is no 'best set'. Much depends on the number of miniatures you want to use, the style of game you prefer and last but not least the fact that some rulesets are rather good to play, but impossible to understand without help. Don't expect a load of rules with army lists, most have some generic entries for the early 20th century and only very few have lists for BoB, and some of these are pure Fantasy.
Here are the rules I know (more or less):
Contemptible little Armies (CLA). The CLA supplement Back of Beyond gave the setting it's name. Never played the rules, barely understood them. The Army lists in the BoB-supplement are pulp-sytle fantasy, but if you like this approach you'll find them useful for other rules, too.
Price of Glory (PoG) - the WW1-Version of Disposable Heroes. Good rules for 20-50 miniatures, though they suck when it comes to close combat (a problem with several systems). Shooting is deadly in the open (especially with HMG), so you need much terrain. Easy to learn, sadly there are only generic unit stats inside. using BoB as a guideline will help a lot to design your Armylists. I used to play this system until T&T arrived, but I think both are equally suited.
Triumph & Tragedy (T&T). The rules I use. You can use them with pulp-style heroes or without, both ways work well. It fills the same niche as PoG, with the same pros and cons. The main difference is the card-driven turn sequence, which gives it a slight edge about PoG.
Warhammer the Great War. 40K with both sides playing imperial guard infantry armies, which isn't bad at all. Strongly recommended for larger games. Again, no lists for BoB.