1. Tuscany -> Italy -> HRE
So this is my Italy campaign, which I finished awhile ago but still stands out as one of my better campaigns. I started out as Tuscany, formed Italy and blobbed on from there. France was my big ally for the first half of the campaign, but she betrayed me twice after I became the emperor by attacking a prince of the Empire so I had to put her down in a few 1v1 wars.

I am the Emperor and have finished every reform except the last. It stands on hold on this picture because I dont't like the way the map looks when the HRE is formed as a country and I hate the boring gray color

Anyway, here is a picture when the reform is passed:
The only thing I regret about this campaign is that it wasn't an Ironman campaign. I thought I was playing an Ironman campaign till it was too late for me to restart it so I decided to just continue it and see how far I could go. Another minor detail is that I unfortunately wasn't able to kill France of in time hehe. Still managed to get a fair bit of land/vassals. Austria is practically the only prince which didn't want to accept the final reform. They are still a vassal of me but I couldn't annex them in time.
2. Byzantium -> Eastern Roman Empire
Another one of my 'victories' was finally being able to get the Basileus achievement. After trying the galleyspam a few times I decided it was time for the diplomatic approach. I managed to get Poland, Lithuania and Austria at my side and crushed the Ottomans in a series of decisive victories. Afterwards I managed to get westernised and started steamrolling through the kebablands, exciling the Ottomans into the Egyptian desert before finally beating them. With the Ottomans out of the way I started taking over bits of Indonesia and India but again, I ran out of time before I could really take over enough land there. It was a fun campaign though and I managed to get a nice Roman Empire in the end. Now it's time to do the same with Trebizond
3. Poland -> Polish-RUSSIAN commonwealth
I started this campaign because I wanted to get some achievements, and there are a lot of achievements which require you to play as Polendball, like abolishing the Sejm and getting into space by using Winged Hussars. It started off easy enough by destroying the Teutonic and Livonian order while maintaining my PU with Lithuania. Then I saw how Muscovy got attacked by the Swedes and all the surrounding hordes at once while losing badly. I decided it was time to end this threat once and for all. In the aftermath I released and vassalised Yaroslavl and Pskov, while feeding them Russian lands. After becoming the Commonwealth I diplo-annexed them and started colonising the Siberian wastes while in the meantime becoming an unstoppable blob. Then I turned my wrath to the Ottomans and finally got my sweet revenge for the Battle of Varna

. Funny thing about this campaign was Bukhara, which became a huge country until I decided it was time to take over some of their lands in order to cut them down. Also France, which got stuck in a PU under Spain till late game is worth mentioning here.
The reason for the pun in the naming of this campaign is that Lithuanian as a culture is no longer accepted in this empire because it fell below the accepted culture treshold. Therefore, Polish, now together with Russian are if I recall correctly the only two accepted cultures in this nation.
I will post some pictures of my Scandinavia and Prussia campaigns soon, but Im still busy finishing them while also trying to get the Komnenioi Empire achievement.