Frontier Brain. Definite Frotier Brain.
That is because:
- They sometimes have legendaries
- You mostly can not use Items in battles
- As DA said, both your and the frontier brain's Pokemon are on the same level
Champions, on the other hand, are much easier to beat cuz :
- They have NO legendaries
- If you have higher leveled Pokemon, you can just sweep through them.
Also, the Frontier Brains are available to battle only after completing the main game. That includes the champions too, except in Black & White
Any way, it's just one's opinion