If you do a search for "New Years Eve" or "NYE", you will find much information, but here is a bit of it...
All of the parks except Animal Kingdom are open until at least 1:00, with MGM having an evening EMH on 12/31 this year, keeping the park open until the wee hours of the morning for Disney resort guests (4:00 a.m. as I recall).
Magic Kingdom WILL reach capacity quickly... if your heart is set on being there for midnight fireworks, plan to arrive early and stay all day. MGM and Epcot will also be packed, but you should be able to get in most of the day, excepting maybe mid-day from around 10:30 a.m. - 2:30 p.m., when they could close the gates until some people leave.
We have done Epcot 2 years in a row... the midnight fireworks there are simply spectacular... the best I've ever seen anywhere.
Be aware that no matter what park you choose to attend, the crowds will be horrendous. I personally don't mind... the atmosphere is a blast and we just walk around and enjoy it. But if you have small children or are expecting to ride anything or eat without ADRs, you may want to think twice. FastPasses are gone for almost all rides before noon. Even with an ADR, you can expect to wait up to 45 minutes for your table. Counter service lines are crazy.
Also, consider that Magic Kingdom serves no alcohol, while Epcot sells beer in glowing cups on the sidewalk... in other words, things are a bit more "boisterous" at Epcot on NYE.