Long ago I read a report by some sniper instructors who used neon flagging tape as a marker on long known-distance ranges. They used their cadre and students as evaluators to see which color was most highly visible. The consensus was that neon pink stood out the most. I don't recall which environment the range was in.

I think just about anything bright will work for your gear, though snow could be a complication. Peter Kummerfeldt says that red paracord works well against snow.

Would greatly depends on your environment, orange is great till you drop it in a pile of autumn leaves.

NEON PINK OR Chartreuse

I use a combination of yellow or orange duct tape. My red based carries have yellow around them, the rest is neon orange. Because a lot of the interior of my packs are black or I might be trying to find the object at night with flashlight, I tend to put patches of neon orange on gear I need but might be hard to find.

Btw, I also put orange duct tape on the back of my tv remote. It always seems to be face down when I am looking for it, so that neon color really pops on my furniture.

White, in the dark.

Depends.....
When I first started doing Search & Rescue, I painted some tools bright orange. Worked great until fall in Maine.

Tried a lot of different colors and found that with my mobile lifestyle, all colors except one would be found someplace in great abundance right where I drop something.

The one that seems to work best is Neon Blue. Not very exciting, but it works very well.

Nomad

I did some research a few years ago on visibility underwater (not quite what you asked) and the best was a black and white checkerboard.

Perhaps a black and white stripe would work well above the water.

For my area and my eyes, neon yellow works the best, especially at night as the flashlight really picks up this color and has saved me more then once from clothes-lining myself on a tarp rope in the dark...

I like neon pink or bright orange for visibility. Supply Captain also has reflective and glow tracers in certain of their para cord colors. http://www.supplycaptain.com/index.cfm?category=6 I purchased some black with reflective tracers to replace all the zipper pulls on my backpack, but haven't tried it out yet.

Not affiliated, etc., just a happy customer.

+1 for Supply Captain. While you're there, get some Type I cord. It is top quality braided stuff that takes care of most cordage jobs with a lot less weight and bulk than the ubiquitous Type III (a.k.a. 550) stuff.