Kind of a tough question actually, as others have noted, because it depends a lot on the person as well as where they live. If the criteria is simply someone just getting interested in the war and simply looking to visit a battlefield park, I'd probably vote for the one closest to them, if there is such a thing. Even if it's fairly small. A site like Honey Springs in Oklahoma, Prairie Grove in Arkansas, or Wilson's Creek in Missouri, are great little parks, for folks who live in this part of the country. Or if a particular battle/campaign has caught their interest, a site or sites involving that. But it depends somewhat on what they're looking to do, and what they're interested in, or so it seems to me.
Probably any of the larger parks could be a bit overwhelming to someone who's coming into it cold as far as knowledge is concerned. Some would probably be a little easier in that respect in others. Shiloh's been mentioned, and while it's probably my favorite park, I'm not sure it lends itself to easy understanding of the battle. If you just want to appreciate a beautiful park, it's an excellent choice. And at its most basic, the battle's pretty simple. The Confederates pushed the Union army back on the first day, the Union forces returned the favor on the second day. The end.
Of course, the details are a little more involved, especially since it's really two battles fought over the same ground on two different days.
And other battles can be like that as well, when you get into the details some. But that's not necessarily bad. A visit to a park, even if it creates a little confusion, can also inspire someone to learn more about the subject, and try to understand it better. So they get more out of it on the next visit.
All that said, I have kind of a Will Rogers take on things, in that I honestly don't think I've ever visited a Civil War park that I didn't like. Some more so than others perhaps, but they all have things worth seeing, things worth learning about, and very often an experience that can touch us in ways that we may not have expected.
Perry