I use Lumecubes for illumination of objects (the ground, fields, cliffs and walls, etc.)
I use firehouse LED to see the aircraft better in the sky, but none of them so far have done more than that (no illumination of objects); though I haven't used the Arc XLs mentioned above, I would wonder if they're still for seeing the aircraft more than illuminating objects.
I have also tried the big kind of lights like Phantomrain mentions above (though I never had mounts for them, just straps, so discontinued). They were the brightest by far, designed for spelunking, but they were really heavy and caused a noticeable difference in aircraft handling.
Chris