I took the question to be asking about specific armies, not nationalities…
Sadly, I do not see any excellent armies or great leadership in all WWI. Technology had made all tactics obsolete and the generals were unable to adapt. As a result, it was a continental wide series of trenches where men died and suffered by the millions simply rotting in the trenches waiting for the order to outrun the machine gunners fire through the barb wire and minefields of no mans land. :cry:
Perhaps the best of the worst was the armies under General Ludendorff in the German spring offensive of 1918.
http://en.wikipedia.org/wiki/Spring_Offensive
After being static for years, the Germans gained a whopping 60 km (40 miles) - an amazing advance at the time - and were then actually able to shell Paris. This was the movement of any significant size in years on the Western front.
BUT-- this offensive badly stretched German supply lines, exhausted them with losses they were unable to sustain, and quite possibly resulted in the allied hundred days offensive and collapse of the central powers. So maybe this was the worst of the worst?
http://en.wikipedia.org/wiki/Hundred_Days_Offensive
That I am unable to decide if this was the best or the worst says a lot about WWI, where even the best armies and Generals were a failure…