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I cleared Celeste with shrapnel in 15 minutes...
I definitely vote for Darkness.
Reasons:
1. The strongest magic.
2. It's enjoyable to watch the royal creatures in the pit.
3. The quests are three times harder than when playing as Light.
4. Why do I love Darkness? Heh, let me tell you. For example, I go into some room, kill everyone with shrapnel, and leave one enemy untouched. Of course, I take 60 provisions with me and go to sleep (in the game). As you know, after you go to sleep, various creatures can attack you. So, what I want to say is that I kill them, go back to sleep, and so on until the provisions run out. :D Then I simply raise the entire army, and as soon as I hit one of them, they all start fighting each other. It's so much fun... You can also just sit in a corner and watch who wins, or you can have fun in different ways, since I have 99,999,999 (using a program) provisions, but it's all just for fun. Try it, and you'll see that it's not interesting to play as Light. :)