Only high-quality H4 halogen bulb will work. Avoid HID or LED bulbs as they are not designed for specific reflector headlight on bike. Will end up with lots of above-horizontal glare into eyes of oncoming traffic. Making for very dangerous situation for you. Unfortunately there's no better replacement headlight, which is usually the best upgrade to start with.
Narva and Flosser bulbs are good. Vosla, Osram Nightbreaker or Philips are good. .
Avoid Sylvania Silvertars, those are crap.
Avoid boutique coated bulbs like PIAA, 4 out of 5 filaments are out of place on those things.
Look at lumens output ratings
All you need to know about lighting:
https://www.danielsternlighting.com/home.html
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Osram Rallye H4 70/65w bulb. Its higher efficiency puts out 2000/1850 lumens, about 50% more than regular H4 bulbs. Got one about 2-yrs ago and it's best output out of them all*.