Where should I apply to college Early Decision?

Let's get our terminology straight. Early Decision is binding; you can only apply to one school ED, and if you're accepted, you must attend (at least, you can't attend anywhere else). WashU has binding Early Decision. Harvard does not; Harvard has Early Action: you apply early and get an admissions decision early, but can apply to other schools (just not early action) and you don't have to attend if admitted. 

I don't think you're getting into Harvard, legacy or not, unless your parent is a very active alum (beyond being a regular donor) or relatively well-known (a senator or representative, chief of a medical institution, or something). Even then it's a long shot. 

I went to an Ivy League university, and I do alumni admissions for them. I've interviewed students with qualifications similar to yours (ranked #1 at a large public high school, high SAT scores, decent ECs and awards but nothing standy-outy) and they were at most waitlisted at my (not-as-competitive-as-Harvard) Ivy League university. 

If you want Ivy, I think you've got an excellent chance at Cornell. Cornell has binding ED admissions (it also has a fairly decent engineering department, if that's the direction you'd like to go). 

At my university, there is no advantage for being a legacy unless you apply binding ED; it actually counts *against* you if you're a legacy and you *don't* apply binding ED. While I'm not sure whether WashU follows this policy, if you like WashU best, why risk it? You're well-qualified for WashU, and they like to pull in students from the East Coast states. 

So, I think you've got excellent chance of ED admissions at both WashU and Cornell. Do either of those feel like "first choice" schools to you? Before Covid, I'd say visit both schools, hang out, attend some classes, and see if you feel at home, but obviously that's not possible now. Talk to your parents, talk to your college guidance counselor at your high school, and go through their web sites thoroughly. 

If you're not really sure about either WashU or Cornell, and really can't figure out a clear first choice, it's better to give up your ED advantage to have more time to weigh your options. If you're considering graduate school, you might be better off at a SUNY for undergrad, if it comes with a hefty merit scholarship.

There are really just too many universities out there for anyone who doesn't know you to make a recommendation, especially since you can't even pick an area of study.  That is a really key factor.  If you want to study engineering, then you shouldn't consider any of the Ivy Leagues, for example.  You shouldn't apply for ED unless you have a dream school that you absolutely want to attend, because acceptance during ED binds you to enroll there.  How are you paying for school?  That's also something to consider when exploring your options.

The one and only one private university that actually has Early Decision, not early action, and that is your top pick.  If you get in ED, it is a binding admission and you will go to that school and will withdraw your application to any other university.  So, you select the one school you really want to go to and won't regret giving up all the other schools for.  If you don't have a school like that, then do not apply ED.  

just apply to a bunch of them and when they accept you then decide