1) Germany- Contrary to djbent's belief, Germany is the most versatile and daring of nations. You have your hands in so many metaphorical pies, its awesome, able to effect the entire board, unique creative openings, fun defence, excellent diplomatic opportunities.
2) Russia is a beast. Its like playing two countries, one in the North and one in the South that are forming a giant pincer movement against Europe. Again you have a good reason to talk to everyone on the board. (Even France. Especially France.) and you can balance a conservative game on one side with an aggressive one on the other, or just bowl everyone over.
3) Italy is the most challenging country on the board. Keeping everyone in balance and still getting off to a quick grow requires some finesse. Certainly not the most successful country, but alot of fun if you like the challenge. Again alot of reason to talk to alot of people.
4) England. England is an easy game to play, its kind of interesting trying to fine tune defensive fleets with offensive armies, and striking the right balance. Placed against highly defensive France and Germany, making gains is tricky, but its easy to play them off against eachother and push for the win over both of them.
5) Austria. I haven't played Austria in a while, but Austria doesn't rank up with the other central powers because of lack of versatility in openings. A good Austria needs to play a cautious game, and a move to Galicia is pretty important. Attacking Turkey is annoying because you need fleets and it takes forever to get them from Trieste, so you need help. I don't know why everyone complains about Italy though, never been attacked by them in all my time as Austria.
6) France. I have never lost while playing as the French, but that doesn't mean its not a lame game. The defensive position means you need to do alot of things wrong with your defensive game to get eliminated early, so I often end up just waiting around for one of the eastern powers to stab Germany so I can get in on a draw. If you get out to an early lead and Italy isn't paying attention it can be a cake walk.
7) Apparently my most successful nation, Turkey. Lack of versatility means I have to play the same defensive system that slowly grinds away at my neighbours. It is said that Turkey should never be able to solo because of stalemate lines, and I don't like hoping everyone is an idiot and lets me win when I play a country.