Who should I play next?

Prussia and France make for good playthroughs.

Austria is a bit tougher but more satisfying if you're able to pull it off.

Sweden is another interesting option.

S-P is easy to unify Italy. Just spam prestige tech to become a GP, and wait for the redshirts to appear.

Well I love to play as Austria, middle of the Europe can expand into Balkans, Rus or even Germany, insane resource potential, Coal and Iron, the bread and butter of Vic 2.

Currently number 1 GP in 1920 or so, 1/2 (switches often) on Prestige, 4th on Industry, 1st Military, just won big great war: A-H(me)+ Germany +Italy vs UK,US,FR and Soviets, too bad the selfish Germany was leader of the alliance and didnt push for my demands, but still good game, colonized half of Africa, 1st on Rubber production about 110.

From begining you face low literacy as your biggest problem, 4% clergy with NF is a must, 1st go for +NF tech (for more clergy focus +plurality), then education up to Darwinism +50% edd eff., increase RP techs asap ofc, after that with military-industrial complex its easy to catch up on those techs, + pick tax efficiency in commerce, those 2nd tier 2 from left can give you very nice boost ot income, can grab Bosnia and stuff from Ottomans, really many many posibilities.

I dont play US, Germany, UK, FR because I find them way too easy.

If you want some fun and challenge go for Austria, China, Turkey, Spain, Brazil, Portugal,...some other south americas or even USCA (united states of central america) can be fun, damn even Mexico.... so many choices

Well, totally love the 2 sicilies. No coal, no Iron. This means you'll have to industrialize by brute force. This is kinda hard but also fun, and unifying italy means trouble with Austria, wich is also very hard but extremely rewarding. Wait for one of the endless austro-german wars to try your luck.

Also, it has a great potential to colonize northern Africa. Chop down Algeria for it's (almost none) iron and Marroco. Odds are you'll face a hard time, but hell!, this is the main purpose. I never played as UK because of the easiness of it's position.

As Comrade110 mentioned, USCA is a hell of great game too, even more challenging than 2 Sicilies.

Prussia, Sardinia, 2-Sicilies are good to learn the influence game. Prussia is easier since you have the options to use war to secure your sphere.

France is a bit too easy to be interesting, at least for me, you have a big population, a nice literacy and it's easy to block Prussia from forming the NGF (Left bank of the Rhine decision let you take a Prussian state for 4 infamy). They're in a good position to colonize Africa with a lot of coastal states to build lvl 3 bases etc.

Netherlands can be a bit more challenging, retake Belgium try to hold your colonial empire and expand without having that many home ports, balance between Prussia and France as allies or foes...

If you don't want to go with Great Powers, I think Two Sicilies and Greece are your best choices. Greece can be especially daunting but it is an enjoyable playthrough.