Most of my games I play as Russia, Prussia or Austria.
Russia has great potential: vast resources, large population and weak neighbors in the east. By 1860ties you can have 4 national cultures (Russian, Belorussian, Ukrainian and Polish) making about 70% of your entire population. You can choose to became a liberal constitutional monarchy (a full citizenship party appears in 1905) or try to preserve the absolutist government and face unrest. Furthermore, once you get Ukrainian and Polish cultures you can benefit from taking Posen, Prussia and parts of Silesia as well as Galicia from Germany and Austria. You can liberate Slavs of central and southern Europe and turn them into your vassals, you can set the border with Germany on Oder... It won't be easy to achieve, but its doable and gives a lot of satisfaction.
Austria is another great choice. It starts strong but in short time is forced to reform or stay behind other great powers. There are many new cultures to gain and with the new hotfix you can keep Hungary after the Ausgleich. Protect yourself against Italy and Prussia and liberate Polish provinces to gain new pops of culture you accept.
Prussia is likely to form Germany which can become the greatest of great powers very soon after it's created. Your potential as Germany is enormous and you can do pretty much everything you want, not excluding beating British navy.
Ottomans are also an interesting choice- you are still a significant power, but Russia is eager to take your territory piece by piece. But if you survive, you will be able to grow from strength to strength and even reclaim your African provinces? Who knows...
Sweden is also a nice country to play. It benefits from high literacy and can gain new cores in Baltic, Finland and Norway.
Greece has great potential for expansion: beat the Ottomans back to Asia and have your king crowned in Constantinople.
If you want something more difficult, Persia and Egypt can become civilized relatively early and they're in Africa/Asia so you can attract more emigration.
Finally, it could be interesting to play the colonial game as France.