Waterloo is good as noted above but I like Campaign Leipzig more.
As recommended to me from the Blitz moderator:
"I own the whole NAP series and from my POV Leipzig is my favorite JTS NAP title.
It is very well put together, includes alot of extra PDF documentation like uniforms and leaders, Scenario Strategy Guide, Terrain Effects Chart, Weapons Effect Chart, Prussian Regular, Garde and Reserve Regiment Listing, and also includes planning maps.
Most importantly, Leipzig has a different feel to it, in a good way, when playing it that I don't "feel" in other titles.
I too, would highly recommend you get Leipzig".
I was glad to take his advice.
Also - if you do, make sure to get Philippe at Bay's mod from GameSquad forums - makes the experience even better !