It entirely depends on you're intent. Sage Rank is more or less if you have a clear custom clan idea in mind and want to create, it comes with a bit more custom slots, increased base HP and Chakra, and the additional cycle slot for subbing customs. If you don't care about the differences, which isn't *much*; go for it.
Legendary Contracts are very nice if you manage to put them together well enough. You can even take it a step down into Mythicals, but those have to be based on an actual mythological creature where as Legendaries can be anything (within reason). My Thunderbirds for example when I took over the CSC, I resubbed them to have a pseudo element called Electro, which is more or less chakraless electric that cant be sealed or absorbed as they are chakraless. It's a pretty nice thing, so def would have quite a lot of room to play with.
Yang Release is also not a bad option, it has a lot of applications you can utilize in customs and battle, geared towards heal, vitality augmentation and even mutagenic manipulation. Its really up to you what you aim to achieve.