My best HRE, started as France.
Couple of highlights:
- did NOT get Burgundy the easy way... had to whittle them down bit by bit, waited for the Netherlands to form, then vassalized French Burgundy and fed them all their cores back
- ended up with the Spanish dynasty on my throne, waited for a disputed succession in Madrid, claimed throne and got Spain under a PU in the 1600s - Portugal had been completely assimilated by them at that point
- Austria got slaughtered, not sure how, but got a mission to vassalize them, which I did, then fed them all their cores back, mostly from Hungary
- became HRE in 1615 with ZERO reforms passed, it was quite a rush and interesting to form it in 150 years
- got lucky with a PU with Denmark in the 1700s
- Bohemia was the only country that did not join the HRE (u mad bro? Yeah u mad), but they had a lot of Polish cores, so I slowly dismantled Poland by releasing and feeding their provinces to Ukraine until I could vassalize Poland, then got them all their cores back from Ukraine and Bohemia - but I could not annex Bohemia until they were down to one province, which happened in 1819 - and I had to break truce two times to do it

- completely forgot about Greece... was trying to occupy all of Western+Eastern continental Europe, but sorry Greece, forgot about you... would have been easy too!