corniilius, Sorry I just saw your question about foods/pet snacks. The answer is not always I have a death pet that likes this storm treat but does love the death candy bar. Most of the time all my pets have likes the fariy cake which gives 4 to power. I would say experiment with different types of snacks to find the ones your pets likes and loves.
Honestly, I don't see much purpose in chasing down the food that my pet "loves", for two reasons.
1. The power that the food gives you is equal the the XP boost that the food gives. So, I can give my pet a snack he LOVES - but it might not give me the XP boost that a snack with more Power points has.
2. No matter what you do, by the time you reach Ancient, your pet will only get a single point of XP per game, and a single point for a snack.
So, enjoy your pet while he/she is young, because as they age, they will only frustrate you!
I think the pattern is that it likes food that raises its best stat (the stat that can be maxed out the highest) or is of the same school as the pet....and loves snacks that have both the best attribute with a school snack.
I haven't done a proper analysis of this though, so it may be off....