It depends on how YOU learn best. I have put hundreds and hundreds of hours into this game, but if my first game had been with Germany I would have been overwhelmed and probably put the game away and not played it again.
My first game was with the USA. It can be very boring, with 4-5 years of peace to start the game but you get to learn how everything works at your own pace and nothing happens until you want it to.
The USSR is probably the easiest country to play. They have more manpower and resources than they could ever use, decent tech teams, and a very focused goal to start the campaign. With a decent sense of history and how to play the game it should be very hard to lose as the USSR on Normal difficulty, even your first game.
If you like to jump in with both feet and learn by swimming or sinking then Germany is a good choice. You will have to learn to deal with everything except Naval combat almost right off the bat. Germany is the most fun to play, IMO, and you you wouldn't get discouraged by failure is a great starting country.