What causes your WORST hangovers?

I’ve been drinking for over 15 years and really started noticing hangovers only since the past few years. One thing I realised is that sometimes I drink a lot but wake up fine, but sometimes I feel like absolute death for the next 2-3 days. Wondering what are the worst hangovers everyone here has experienced and does anyone know from personal experience what was the variable that actually caused it to happen?