France: With three potential neighboring allies, France has diplomatic options that rival Germany's. The strategy is different. As Germany will be as proactive as possible, setting the world events and playing one power against another, France, with two guaranteed neutrals can wait a year, or two, before deciding what to do. Often times France can lose one or two home supply centers and still be in a diplomatically viable position, and it's in this reactionary diplomacy that a good French player can really shine. Playing France well takes an almost contrarian world-view, one that's not for every player. But if you don't mind first defending, and then attacking, France can be a great country... and a lot of fun. Italian play can be similar, though the advantages are not nearly as good.
Another favorite is Germany, for the reasons Akroma stated. Depends on my mood. Though if you want a power trip, England's your choice.