Couple of favorites:
Corruption of a Champion, its 3D and the initial transformation is pretty quick but it fits some good places:
https://e-hentai.org/g/1035517/d86f7c751e/
[Ruuen Rouga] Tamashii no Kusari
All time favorite, slow semi-voluntary process. Story is interesting, there's wad am English version out there under the title Soul Chain" but I couldn't find it at the moment.
https://e-hentai.org/g/846142/ba63124154/