Russia, Germany, France, Italy, England, Turkey, Austria.
Russia-- maybe it's that E/Gs rarely form against me, but this is my most successful. R/I alliances and many expansion opportunities make this country the most appealing for me.
Germany-- also many expansion opportunities in the middle game. And Austria will *always* want to ally with you.
France-- hard to beat, but less distant than England, 2 quick builds. Unless E/G/I attack you on turn 1 of course, which has happened.
Italy-- not an easy country for me, but successful I/R or I/G alliances can be very powerful.
England-- hard to beat but hard to make progress. And if someone does get into one of the English seas England has only a few, painful years left.
Turkey-- none of your neighbors really like you usually! And progress is difficult, especially with your best alliance being so feared.
Austria-- I always get attacked early, especially by Italy.