I'd probably say:
1. England: has excellent defensive position, excellent chances to be in a great position to perform a stab on Germany and France later in the game, two fleets from the start, nice opening moves. England requires diplomatic skills to strike deals or set up alliances with your three nighbours to grow faster and obtain centres and strategic skills to perform power moves with fleets and armies as soon as 1904. What's not to love with England?
2. Germany: I don't know why but I get to play as Germany a lot of times, but I really like it a lot. You have to talk with everyone, everithing that happens on the map is your business you have six (SIX!) neighbours and you're straight into the action from S01. As Germany you can play the Bismarck-style diplomatic honest broker talking and convincing everyone to behave properly as well as the warring blood-thirsty Kaiser going for the blitzkrieg in the mould of Wilhelm II. Sometimes it's hard to have everything in control, but versatility and spotlight make Germany an entertaining power.
3. Turkey: basically a poor man's England, I would say, but really likeable anyway. The defensive structure of turkish homeland allows you to be as aggressive and attack-minded as you can be in search for quick growth, keeping in mind that Turkey can be a dreadful force on the land and on the sea at the same time. However Turkey could result a bit predictable and there's always a chance to end up confined in your corner very early in the game. Anyway a very enjoyable power to play.
4. France: lots of opening strategies, extreme versatility, a huge number of free centers waiting only for a french unit to get there and claim them, France is a nice power to play and easily the strongest power with Russia on the map, but in my opinion lacks something. French prowess from early on in the game is too easy to obtain. Basically you have two centers for free and a good spot at a third if England goes full Churchill opening and Germany pushes for Denmark (a situation that often happens). It's always nice to be a true powerhouse from the beginning, but i enjoy more the thrill of talking and planning moves to achieve centres.
5. Italy: extremely situational, Italy could be the best or the worst power to play in this game. I like a lot Italy's ability to play on both eastern and western triangle, I think it takes a lot of aggression to be a succesful italian and when this happens, if you manage to grab two or three centers at the end of A02, then Italy could be really entertaining to play. Italy needs the same attack-minded people required for Turkey in my opinion. The only problem is that your defensive position is, unlike Turkey, extremely weak, so sometimes things won't go as planned: sometimes Austria will turn on you in A01, sometimes Turkey will deploy fleets deep into the Mediterranean before you can, and that is the moment when playing Italy turns really sloppy, slow and really unamusing. You're basically everyone's second best friend out there, and everyone of your allies will look at you as one of their cheering little henchmen. It's easy to be bullied by Russia, France or Turkey when you're Italy. Maybe a second fleet could make Italy more enjoyable to play...
6. Russia: I find France too "easy" to play, so what could i possibly think of Russia? As I said, it's great to be a powerhouse, but this is too much. You have one of your starting centers in the western triangle, SWE and RUM easily achievable, four units from the start (and four-unit-per-turn building when you start the rolling), Germany going west most of the times, Austria desperate to give you everything for a nonaggression, Turkey as a fantastic partner in crime just in case you want to start a Jug, Italy always eager to listen in case you ask for a wintergreen and even possibly England for a Top Sweep. Come on, what else do you want?
7. Austria-Hungary: Austria could be fun to play, could be boring to play, but odds are it won't be long. Austria is the only power on the board with his future resting in other powers' will. You're basically a big gory piece of raw meat in a shark tank and the only neighbour you can trust, Germany, plays primarily on the western triangle, leaving you alone in a room with a big bad bear and two ambitious and aggressive wolves on both your sides. I like to earn my centers, I like to use personal and tactical skills to grow and i don't like when it's just too easy, but, although always fascinating, playing as Austria is too much of a task to be an enjoyable thing.