Magic Kingdom-right in front of castle, sit on stone benches facing the castle, someone from your party will need to go atleast an hour early to sit down with maybe bags next to them to save a second seat, the rest of your party will need to arrive no later than 30 min prior to parade start to avoid crowds and to make sure you get seats
Epcot- hands down watch illuminations from Mexico, sit on one of the two benches on the lakeside right after you cross bridge leading into Mexico, again one person will need to arrive atleast one hour early etc., I know this is not a parade but these fireworks are great and i have watched them many times from other locals and Mexico is the best, I dont know about the other parades