With the help of an ability, the winner would be Mega Venusaur with 2 weaknesses of Flying and Psychic.
Without an ability, they would be the Ferroseed line and Kartana (Grass/Steel) with 2 weaknesses of Fire (4x) and Fighting.
Also, they would be the Alolan Sandshrew line (Ice/Steel) with 3 weaknesses of Fire (4x), Fighting (4x), and Ground.
If you want triple-types, Grass/Steel/Fairy with 1 weakness to Fire (4x) and Ice/Grass/Steel with 2 weaknesses to Fire (8x), and Fighting (4x).
Source: knowledge of Dual-type Chart and This (especially for the triple-types)