If your subject line were the only question, I’d concur with others who said Area II. We have the most events in a compact geographic area.
The rest of the post reminds me of my borderline-obsessive quest, while pregnant, to find the ONE stroller that I could use for every purpose at every age until my kid outgrew strollers. Not possible. Also not that necessary.
I didn’t see what your career path is, but I will say this: it’s very difficult to find a place where there is no commute to riding and where the career opportunities are thick on the ground. If you’re going into technology, then the I 270 corridor in Maryland, which is close to a number of great events, will work.
Baltimore and north of it are wonderful places to live, but the job opportunities are Not. So. Much unless you are in the health field. I have friends in extremely sought-after fields who regularly try to find something near Baltimore and consistently get head hunter calls for Silver Spring, NoVa, and DC.
There are places that are fairly recession-proof and have big local industries-- like DC-- and then there is everywhere else.
That being said, much like the stroller, you don’t need to find a city that provides college, eventing, and career all at once. Most people don’t stay in their college towns. There’s really no reason someone has to stay in the same event barn after college that she was in during college. Sure, if you were focusing only on riding, you might want continuity-- but you said you have a non-rural career path in mind.
Go to the place that gives you the best balance of education and riding experience you can find. Then go to the place that gives you the best combination of work and riding experience you can find. Five will get you ten they’re both in one of the eventing-dense areas between NJ and FL, but if you try to lock yourself in to finding all three, you could end up mediocre all around.