What's the best fertilizer to make hibiscus bloom more?

There's no "best" fertilizer that works for everyone, because it really depends on the soil you have and how much care you can give it. But I can tell you what works well in most cases.

Hibiscus really does like phosphorus and potassium, so look for a balanced fertilizer with those nutrients highlighted - like NPK 10-52-10 or something similar. If you have access to it, use slow-release granular fertilizer, because then you apply it once and don't have to keep watering with fertilizer every week. Frequency matters a lot: during the flowering season (spring and summer), you apply fertilizer every 3 or 4 weeks. In winter you can space it out more or even stop.

Now, fertilizer alone won't work miracles if your hibiscus isn't getting real sunlight - it needs about 6 hours of direct sun a day. Consistent watering is also essential; the soil has to stay moist but not waterlogged. If you're using fertilizer and the flowering is still weak, the problem might not be nutrition, but rather light or air humidity. It's also worth adding some organic matter to the soil from time to time, like humus or homemade compost, which improves nutrient absorption.