@Kediil that depends on what method of raising your partners you are doing.
While it is true that putting all your Potential fruit into one or two Partners will make them insanely strong, you will still have those weaker Partners that can be more easily picked off.
But if you even out the potential, you will have more evenly raised partners. This makes it harder to pick off your partners. But you won't have that one or two who are stronger.
Every method of raising has it's own pros and cons. If you are just looking to have the most possible power, than yes, the best method is the higher level partners.