Portugal is a good choice for experience players who dont want pressure of being in middle of Europe cuz it usually means endless wars. If youre a total noob with no clue of how colonization and other key mechanics work, Portugal is not a good country. No matter who you play as, you will be dragged at wars, AIs will come at you sooner or later, you can count on that.
The best 3 countries for a beginner are the following:
Castille - the biggest country in its immediate neighborhood, nobody can beat you one on one (bar France), and you can practice warfare, colonization, you even have a country unification to pursue (Spain).
France - the European Behemoth, or Big Blue Blob as its known from early EU3 days. What to say about France? It can do everything, and also excel at everything... Just dont allow any of your rivals to get bigger than you and youll be just fine.
England - last of the 3, on its own it has the best noob position to play around, however in EU4 it will start at war with France (the end of HYW) and that might be a bit too daunting for a new player. Apart from that, you can either decide to push into France (this will be hard and require experience) or you can give up European possessions and focus on colonization and unification of Great Britain.