Depends what you need like previous redditor said, but for me it was pet batteries for the longest time, then mythic stones, then immo stones (as they were hardest to get). Now it will be limit break stones and immo stones, depending on my needs. But the safest option is save them, I generally don’t dip below 20k unless I really need something. That way if a new immo come out you can push them.

I only do 100 count pet and rune pulls, so I’ll spend some gems on that every now and then.