what´s the best crossover?

...and I do not mean brands.

Here is my question:

I own a set of infinity kappa 6.5" ,each speaker comes with a crossover but what happens if I want to hook an electronic crossover ,say, a kicker or clarion???Well the brand doesn´t matter for now...

Do I must connect both crossover??? or discard the one that comes with the speakers???

That depends.  If you are trying to put together a full-active biamp system then yes, you would replace the function of the passive crossover with the electronic one.  And you'll need additional amplifier channels to power each speaker.  If you are just using the electronic crossover as a high-pass for the mains amp and a low-pass for a subwoofer, for example, then you still need to use the passive for the component set.

In general, unless you really know what you are doing and have the proper test and measurement tools and skills to set it up properly, you are better off using the passive crossover that comes with any component set simply powered from one amp channel.