Best pet for a destro in arena?

Felhunter or Succubus ?

It depends of situation and against whom u are playing. Felhunter is good vs casters and Succ can be used against melee.

There is no "best". Roll with a succubus and end up against a caster team and she will be useless, apart from the occasional knockback or charm. Roll with a Felhunter vs a melee team and it will be just running around doing nothing.

If you are talking wotlk then it depends on what you are playing but since you are asking this question you probably aint playing 3s.

For 2s the main comps destro have are:

Destro/ Heals, Pet you start with is Succubus.
Destro/ Fmage, Pet you start with is Felhunter
Destro/ Ele Pet, you start with is Succubus.

For some comps you face you should swap out the pet you started with but what i wrote above is a rough outline of what the high rated players use for pets.