During the healing process? Absolutely burns, no question. All those things suck while they're happening and in the immediate aftermath, but for sheer horrible miserableness that just never seems to end, or even get any easier to deal with, severe burns just can't be beat.
I'm not sure why burns are broken down into two categories. The immediate consequences are different, but the healing process is the same for them all. And even if you do want to break them down, why heat and radiation in one category and electrical and chemical in the other? Heat and electrical seem to have more in common with each other, as do radiation and chemical. The only thing I can think of in common for heat and radiation is that they're the two most common kinds of burns, and there's no quality that electrical and chemical burns share that isn't common to all burns to the best of my knowledge.