Maximum civilian deaths, obviously nuclear weapons. Maximum deaths with minimum blame, probably very close to what Israel is currently doing. Use an indeterminate mix all kinds of various technological weapons from a variety of vendors and a variety of doctrines. Ensure that no one person is to blame for any specific atrocity, always suppose that "better" weapons could have lead to a "better" outcome. Otherwise bomb indiscriminately for as long as you can get away with it.

Modern day weapons from the west, probably carpet munitions. I don't think they're using them, but I could be wrong. 🤔

Worse I'd imagine to be chemical weapons ala Asad.