Best way to spend your crystals?

Refreshes. And sorry, I'm too jaded by the game and the questions been asked too many times to give you a long elegant answer.

Suffice it to say you can get a number of rewards from refreshing energy and farming (and not just the one specific thing you are farming) such as currency, mats, credits, other gear, etc that all add up to pushing you forward.

Then when you have a developed roster? Refreshes are still better.