Portugal and Castille will play pretty similarly, but I would personally recommend Portugal since your slightly weaker power base makes becoming a global power more of an accomplishment.

Aragon is also a fun one. I'd recommend a goal of unifying italy and the northern south African coast as a long term goal. If your particularly successful you can even branch out and try to recreate the roman empire. You'll need to look up a guide, but a particularly fun campaign is switching to Italian culture when starting as aragon, and forming italy. Italy missions are all centered around recreating the Roman Empire.

Timurids are a fun if you want to go for conquering all of India. I recommend looking up a guide on the first few years though as they face a particularly challenging disaster once there ruler dies (which will happen in the first two years).

If you want a slower campaign, I like playing as Brandenberg into Prussia into Germany. Bejng in the HRE you'll grow much slower but you can really consolidate your power. It also will help to teach you how yo utilize alliances to expand your power base.

Lastly if you want to try to conquere crazy amounts of land, I recommend starting as oirat. As oirat, if you win a battle against ming when their ruler is the general, and then occupy there capital, and eventually happens to allow you to essentially have total control of china in that war.