[Gallagher]

There are too many factors in that to really give a proper answer, and even the elder wand's status could be debated, depending on the circumstances involved. There's so many different things that matter. The combination of wood to core is important, yes, but so is the craftmanship involved, the kind of magic being used, the wizard that owns said wand, the bond between the owner and wand... There are so many variables that, while one can say that certain combinations are better than others, I would be hesitant to say that there is a best and worst.

If you really want examples, though, a unicorn and apple pair would be useless to one that does Dark Arts. Chestnut and any more volatile core like veela, kelpie, or kneazle would make me cringe, too.