To get to Peninsula Valdés from Buenos Aires, you have a few travel options:

Peninsula Valdes from Buenos Aires by plane

It´s the fastest and most convenient way to reach Peninsula Valdes from Buenos Aires

First, Fly to Trelew (REL) or Puerto Madryn (PMY)

Trelew Airport (REL): Larger airport with more flight options. About 2 hours from Buenos Aires (AEP or EZE).
Puerto Madryn Airport (PMY): Closer but fewer flights. About 2 hours from Buenos Aires.

Then, book a transfer to Puerto Madryn or Puerto Pirámides

From Trelew (REL):
Bus or private transfer (~1 hour) to Puerto Madryn.
Another 1.5-hour drive to Puerto Pirámides (the main gateway to Peninsula Valdés).

From Puerto Madryn (PMY):
Direct drive to Puerto Pirámides (~1.5 hours).

Another option is to take a bus service. It´s cheaper, but long.

Going from Buenos Aires to Peninsula Valdes by bus

Long-distance buses from Buenos Aires (Retiro Bus Terminal) to Puerto Madryn (~18-20 hours).

From Puerto Madryn, take a bus or tour to Peninsula Valdés (~1.5 hours).

Recommended companies: Don Otto, Andesmar, Via Tac.

By Car 

Ideal for road trip lovers. It´s a long option

Drive from Buenos Aires to Puerto Madryn (~1,300 km / 14-16 hours). Then drive to Peninsula Valdés (~1.5 hours).
Good for stopping in places like Las Grutas or Viedma along the way.

So, what´s the best option to go from Buenos Aires to Peninsula Valdes?

The best option is by flying to Trelew or Puerto Madryn and then taking a transfer to Puerto Pirámides is the most efficient way. If you have time, a road trip can be an adventure.