Well, there's no straight answer to that I don't think. Taelia's quests in general give out the best items I believe, but she can ask for some really expensive stuff in return. And one time I spent 15np and the Esophagor gave me a 93k plushie. To ensure I get a profit I always do:
*Taelia's quests that cost under 3k
*Kitchen quests that cost under 2k (but that's when I'm hoping to get stats)
*Esophagor quests that cost under like... 100np (with his quests they are usually under 100np or over 5k (and more often than not they're over 100k) so yeah, just do the easy ones)
*Edna's quests that are under 200np
Oh and I never bother with the Brain Tree, he requires you to do 2 Esophagor quests in order to get his reward, and more often than not if you get a cheap first quest, the second will be extremely expensive.
Hope I helped
