Best Rock Texture for this situation?

Displacements, dude, you need them.
You need a shitton of them.

For huge walls, I'd suggest using a simple rock texture, that doesn't have nearly as much.

There were some in the bulletcrops pack that i think could work pretty well, but other than that,
nightfall/cliffwall and a lot of the textures in the /rockwall series seem good.

Well, the thing is that the nightfall one is a re-skinned version of some of the /rockwall textures. I wasn't sure what style your map was, so i included it for safety's sake.

I think the texture you have in place currently is just fine. It may look bad now, but once you turn them into displacements and you start getting some shadows going on it'll look a lot better.

Along those lines, take a note of where the sunlight is coming from in your map. If the sunlight is hitting the wall head on, you won't get any shadows. If the sunlight is coming in from an angle, you start to get some nice texturing.

It's important to think of now because changing it later on requires rotating your entire map by 90 degrees, which can cause a whole bunch of headaches with brushes not being on the grid.

(technically you can just change the angle of the sunlight to whatever you want, but then the map lighting won't match the location of the sun in the skybox which looks kind of weird)