I just moved on from EU2 to vanilla EU4 recently, and upon hearing that the United States can be created by simply releasing and playing as a colonial nation with as low as admin tech level 10, I am curious which colonial power I would have the best luck with creating the US as soon as possible from a 1444 start, without using cheats, obviously.
I would go for England or France.
If you play with the aim to becoming the US later on, it makes no sense to do this as Spain or Portugal since the trade generated on the NA East Coast can not flow towards your centers of trade, instead ending up in Bordeaux or English Channel. Thus the Northern European nations are the better path to your goal.
If you want to be close to historically accurate England would be the best choice and it's also the easiest route given the shorter distance in terms of colonial range and more naval focus in traditions.
1) You can start at a later date and play as United State. There is an achievement for that. Start in 1776 bookmark and hold all of your cores and be at peace with England.
2) The longer way around is like you said release a colonial nation.
You can go with either portugal or castile or England or French. Or any other colonizer with a huge tax base really. You just need to pick exploration/expansion and quality (for the policy bonus to settler growth).
Really at most the choice comes down to what culture do you want as United State?
Do you want a challenging Europe start or easy one?
Notice: DO NOT use random new world as that would disable the form-able decision for United State.
You may not want to stay as catholic because breaking the Treaty of Tordesillas repeatly as Catholic will hurt your settler growth. Switch to either reformed or protestant.