I personally find the NAP series to be my favorite John Tiller Series, with the ACW series running a very close second.
In case you are not aware of it, JTS offers a free NAP demo which can be found here:
http://johntillersoftware.com/Demos.php
The demo has a handful of Waterloo scenarios, which will allow you to get a little taste of how the series is.
Waterloo and Leipzig are my two favorite titles and are also very popular titles with several of the Blitz guys.
As with starting out new to any John Tiller series, it is always suggested that you select a title that you have interest in however, you may find that it will be easier to find Blitz PBEM opponents if you have Waterloo.
If you have any questions, don't hesitate to ask.
-------------------------------------------
It appears I am a slow typer!