Is there a good way to make the spot/floodlight sprites always align on an arbitrary axis? I have a few flood lights that point at an oblique angle, and I can't get the light beam sprite to align properly.
Valve uses a particle system for their lights. I dont know its exact name but just check one of their maps and youll find it.
I'd definitely recommend point_spotlight. Just angle it to the same as your light source and it should handle all the sprite orientation/visiblilty itself. If you're looking into the spotlight from the front you see a halo, from the side it's a light beam and other angles mix between the two.
They also use particle systems. I think thats only in the later maps though. I think it depends on the area though as particle systems are heavier to use. But im 100% sure they use them as i copy my lights (and edit them ofcourse to be ideal for my map) and particles from other maps most of the time.
Also make sure it's not inside anything, like a spotlight prop for example, an easier and more efficient way to do things would be to use Radaka's Lighting Family. A great Resource made for copying and pasting premade lights into your map.