I choose TacOps, its detailed and easy to pick up, with a very intuative interface. So the person I would be hypothetically introducing to wargaming would be able to concentrate on the fun of the tactics whilst the combat system handles the other little niggly details that comes out in wargaming.
You can also see results very quickly in TacOps which is what a new person needs to get into wargaming
I know you said one wargame but to advance the person's interest I would let them play POA2, and show the extreme detail levels that you take a wargame too.
If someone is going to like wargaming I believe they need to see detail and realism with out spending a long time to get results.
Personally if I were a newbie to wargaming, I would be quite put off by the likes of POA2 TOAW and Decisive Action, because the initial workload is way too much, IMHO.