I would say Russia…
It is very true that Russia only has to worry about a few territories and doesn’t have a lot of buying opportunities. However, Russia is a tactical nightmare for anyone who wants tan easy game… Don’t expect to gain a lot of ground much less win as Russia unless you have REALLY good Allies. They also have a terrible income… But as Keredrex said, you only have to buy land units pretty much.
For simplicity’s sake, yes Russia is easiest. But from a tactical/“winning”/strategy standpoint, it’s pretty hard.