The best park to visit today is Epcot. You should also be ok to visit Magic Kingdom, Hollywood Studios, Animal Kingdom, Typhoon Lagoon and Blizzard Beach, as we expect average crowd levels there.
Blizzard Beach has extra magic hours in the morning, so unless you are staying on site, we recommend visiting another park as it will be slightly busier until around 2pm. There aren't any evening Extra Magic Hours planned at any park today.
If you're staying on site and know that you'll be able to get to the park for when the Extra Magic Hours start, we recommend going. Over 70% of guests staying on site end up late for morning extra magic hours, they are only worth taking advantage of if you are there for the EMH start time. If you're not staying on site, you should try to avoid parks with extra magic hours in the morning. If the park has an evening EMH it should be quiet the morning after, so try and visit then.
If you don't plan on watching evening fireworks or other events during the day, try and visit the most popular and busy attractions when said events are on. Since a large number of the parks visitors will be at the event, you should find the rides are fairly quiet.