What is the best class to pve and pvp with

That one. They all have their strengths and weaknesses: warriors can take damage but not always dish it out, mages can dish it out but can't take it, shamans have more power in a group than alone, druids can do lots but can have the "jack of all trades, master of none" syndrome, Priests really rely on groups, Death Knights are great alone and pretty good in group, Warlocks can be fun and deal massive damage in PVP... It's about using the strength of your class against someone else's weakness; the only way to ever win in a competition.

You do the same in PVE - use the skills you've got in ways that the enemy can't counter: as a warlock big critters can be destroyed with well placed DOTS and fear, Priests can heal themselves faster than alot of creatures can pump out damage, Paladins can heal well but often are better against groups of creatures (mana efficient), Shaman can play to the weakness of the enemy by the choice of totems, Mages can fire some damage, then blink and frost nova to keep away from the mob, Druids can flip forms as needed ...

The best players don't all have the same classes, because they either adjust how they play to match the class they choose, or they choose a class they can play well. If you have a hard time playing a warrior, try a mage or a priest, if you hate dying over and over because you only wear cloth, pick a DK or a warrior... play to your strengths.