Which heroes can escape Zarya's ult?

Update: As of July 2017, Zarya's gravitational surge now disables all movement abilities of affected heroes preventing their escape.

Some abilities, however, do remain unaffected, as they are not true "movement" abilites:

• Orisa's Fortify is designed to let her escape effects like this.
• Reaper's Wraith Form isn't a movement ability as much it is an immunity ability.
• Moira's Fade works like Reaper's Wraith form, granting immunity and escape.

These heroes have the capability to actually escape a Graviton Surge after getting trapped in one:

Tracer can use a Recall to escape, or two quick Blinks.

Reinhardt can use a Charge to escape.

Genji can use a Swift Strike to get out of it.

Reaper can use Wraith Form to escape.

D.va can use her Boosters to escape.

Widowmaker can use her Grappling Hook to escape."

Source

Per this guide (which I personally found very helpful, it's far more than just tactics, it explains counters in-depth as well along with specific information about each ability), those are the only heroes currently (as of June 3rd 2016) capable of escaping, and he specifically listed how he thought it was weird that winston and pharah were unable to escape considering the strength of their jumps.

As of the July 27 2017 patch, the answer changed. Now all movement abilities are blocked.

So now there are the following ways to escape:

• Orisa: Fortify
• Zenyatta's Ult (immunity)
• Mercy's Ult if there's someone in range to rez (short immunity)

Zarya

Graviton Surge
    Now disables mobility abilities on all affected targets

Developer Comment: In the past, the abilities that allowed heroes to escape Zarya’s Graviton Surge felt fairly arbitrary. We’re cleaning this up and increasing Graviton Surge's overall power by rendering all movement abilities unusable when an enemy is trapped.