red pixie cause of it's unity giving you +150 magic damage at Lv95.
and a blue pixie as belt pet, Lv90 = 10% magic damage.
Then you can switch when you need a faster mp regeneration and put the red pixie as belt pet while that your blue pixie is out.
The real best pet however would be either a white dragon or an angel.. but good luck getting those, there's not any white dragons on any of the us servers yet and an angel.. well a tamed one is over 1billion.