its combination of following:
- heavy wiring to carry current without melting and cut-outs
- a good quality h4 bulb. You can easily get imported ones from good reputed car shops
- metal reflector with glass beam. This headlight, which I sourced, is from a gs400. Any such headlight will do. Dont worry about fitting, a denter can easily fit those
Although mine was bolt on fitting
Gs150 already has a 8 amp battery so I didnt need to change 
I hate using HID without proper HID reflector as it will only blind the traffic while you will still not be able to get a nice beam. And as brother fazalwahab11375 has correctly pointed out, it is un-ethical too. But if you have $$$ to spare then get a proper HID housing and reflector with a good quality HID kit for motorbikes (both of which will probably need to be imported). Then you will have more bright light than any halogen bulb (of similar wattage) 