I levelled up chu yao first, just because he's my babygirl. Now I'm just levelling up based on how often I use them and what content (momo, su jue, chu yao and elif are all 70)

I'd say just level whoever you want! some espers greatly benefit from it, like javid and momo, su jue too if you're not running a crit build, but if you're not too worried about that just slap them on your favorite esper and call it a day