My list:
1. Germany. The traffic cop of the Great European Highway. Very important in checking both R and F ambitions, and the home territory of Munich, which is Mister Eighteen in any number of solo scenarios. Common mistakes include overly trusting E/F/both and (my favorite) attempting to solo as a 6 SC power in 1903.

2. Italy. Not a power all love to play, but Italy is very important to how things play out in France and the whole Med theater. Also has a big say in both Tun and Mun.

3. Russia. A great Russia can be the conductor of both the western and eastern theaters. A decent Russia talks to at least three other powers subtantively and watches events closely two or three provinces away from his or her own.

4. Austria. On the Great European Highway, but staring down a semis from the get-go.

5/6. England/Turkey. These powers can get stopped cold early due to alliances of convenience even when played by really good players. Even very new players can have a lot of influence on the board as 3-4 SC T or Es once they figure out the basics of naval orders.

7. France. Has the best chance of surviving some early missteps, hard to kill off, and plays a big role in board flow even if reduced to not much more than MAO.

Austria if I'm Italy. Italy if I'm Austria.

Russia. A Russia who has no idea what he's doing can create more chaos on the board than any other power. Though Germany is a close second for the same reason.

Conversely an Italy or Austria who has no clue what to do will usually be eliminated quite quickly and thus do little damage.

An England or a Turkey who is learning the game will probably survive longer than Austria or Italy, but will also have little impact beyond their immediate borders, and thus is also not really a concern.

An inexperienced France can go either way. Usually France's way forward, particularly at the beginning is pretty clear. But given it's position on the board a sufficiently determined idiot or newbie can still wreak a respectable amount of havoc.

Russia for me is the key. A weak Russian player simply implodes and creates a vacuum and absolute havoc. You see this mad scramble for his SCs so england and turkey and germany can all come to blows over what are essentially very small gains. I like to play with a strong Russia.