Easiest:
Spain in Americas
Ireland in Brittania
Spain in vanilla

And right now, I'm having a great start as the Danes in Teutonic. Everybody else has picked a sparring partner (HRE v. Poland, Teutonic Order & Mongols v. Lithuania/Novgorod), so I got plenty of time to mop up the Norwegians. I own all of Scandanavia, the big island off of Sweden and three regions in northern Russia, plus my initial territories down to Hamburg. I just declared the Kalmar Union, which gained me a biggish army and two navies, and now get to decide who to beat on. Because I've been allied w/ both Novgorod and HRE from the start, and because they've both been in full-scale war on other fronts, I have nothing but garrison troops in front of me. ... Either way, I'll have three of five Hanseatic League cities inside of 10 more turns.

Hardest:
Russia in vanilla (I played it twice ... the first time the Byzantines got clobbered out of the blocks, so I was facing Roman Catholics from the Baltic to the Black Sea almost at the start, and an aggressive Poland meant I was at war with a Catholic faction from the start, which meant every Catholic faction disliked me on contact. By the time I got the upper hand against Poland/Hungary/Venice/HRE/Denmark to the west/southwest, the Mongols came knocking at my lightly garrisoned back door ...
The second time, the Byzantines at least kept the Magyars preoccupied and it was easier, although the Danes got Stockholm early, which really reduced my income.)