In the gen 6 games there's an npc who allows you do an inverse battle where type match-ups are reversed (i.e. Bug becomes weak to fighting but resists fire etc.) So during an inverse battle, which Pokemon has the most weaknesses?
By looking at the Dual-type chart and counting the resistances/immunities of the Part-Steel-types (the type with the most resistances), I can deduce that the mons with the most weaknesses in Inverse Battles are the Magnemite-line and the Honedge-line with 12 weaknesses each.
Also, Normal/Steel has the same number of weaknesses as those listed above.