Cars - For Car Enthusiasts

I don't know for your specific model, but flashy aftermarket housings are notorious for having worse headlight beams than oem.

If you truly want best possible, that's probably a projector retrofit. You can diy for about $500 (+headlight housings) or pay specialty shops around $1500 for a basic retro. This puts a projector pod (no, not some LED flood light, an actual headlight unit) inside the plain housing.

If you have the 99-07 style Sierra with a 9006 low beam, 9012 bulbs (and wire cutters to trim the tabs) is a reliable bet. Same wattage, but an infrared coating nearly doubles the lumens. It's a shame these didn't stick around. If it's the 07+ style with H11, you could consider an H9 as long as the low beam has a glare cap. It's a higher wattage bulb, but not by much. 65w vs 55w I think. It doesn't seem to be more than the housings can handle, typically. Could be more than 30% more light. No idea what your aftermarket set has though.

Aside from that, there's not much that actually improves headlights, assuming they're in good condition (lenses and bulbs). People love throwing HID/LED bulbs in there and think they're so cool, but they actually suck. Majorly. People get wowed by puking light in the first 50ft. That does jack squat at 40mph. It's hard to gauge how well light throws down the road, so they don't notice the drastic reduction in sight distance. Meanwhile, it's guaranteed that the beam is now glaring at every other driver. No, you cannot just aim them down. Not if you understand anything about glare, hotspots, or beam distribution. You want good coverage across the beam, a central hot spot at the very top of the beam to throw way down the road, and as little glare as absolutely possible. But people are naive.