I'd vote for Spain but that may be "current campaign bias" because I'm doing so well with that faction. They have England/Scotland's advantage of geographic protection, but they're right there in the Med to take advantage of sea trade, and they have much easier access to the Holy Lands on crusade. They have a close enemy (Moors) that can be attacked with no consequences to your relation to the Pope, and you can ignore the Mongols and Timurids if you want, just sacking Jerusalem and leaving when convenient.

When it comes to units... they get one of the 3 most powerful navies in the game, they get the incredibly versatile Jinete light cavalry, and a great gunpowder unit in the later game (musketeers). But I think the main advantage is the map position, not the units. Portugal is easy to wrap up early, then you can seal your border against France while mopping up the Moors. From there, you can go anywhere and only deal with two or three fronts at most, instead of being surrounded.

About the only disadvantage I can see to Spain so far, is that they don't have great infantry or archers in the early game, so you have to punt with mercenaries. And conquering the African provinces bogs down the rate of conquest a bit, due to the very long travel times down there. Once Timbuktu is secure though, that's (literally) a gold mine, economically. Spain is the ultimate faction to play if you want a "safe" home base backed up against one corner of the map.

My next campaign will probably be Turks, to see what I can do in a much less secure position.