Light is so much better than all the other starting spirits it's insane. Breed / Create a bunch of Fairies (or Seedling Fairies for Druid) and you just have infinite free XP potions. Fairies have Photosynthesis so there's basically no limit on the number you can have, and they'll practically pay for themselves just by fishing. If you ever need money, just hire a soldier and instantly level them up using the ten billion XP potions you've gotten.
Druid is so easy with this playstyle that I won with him on max difficulty with zero effort (no hard mode or hero, though, it was my first win). Five maxed out Seedling Dragons by like 20 days in. At some point I just started breeding Dragons en masse and selling them to the creature guy for 3k gold each. Can't imagine how easy it is for Siena who doesn't even have to bother with breeding the Fairies. You pretty much never even need to do anything, just sit in your base breeding / fusing for better genes and then level stuff up instantly when you actually need to fight.
Stone gets an honorable mention too of course but it's nothing compared to the gains from Light.