I think in a vacuum Ziggs is probably the worst but once we factor in the cost I think that Viktor is the worst unit in the game. He needs so many items and synergies (scholar/arcanist) just to be "playable" and you're probably better off investing those items on any other 5 cost carry and probably even some four cost carries (like Fiora).

My worst unit at each cost:

1: Ziggs. You can argue Illaoi but at least she gives you early 4 bruiser or merc 2: Tossup between Zilean and Zyra but at least Zyra has two good traits (Syndicate and Scholar) 3: All the three costs are actually pretty good. But if I had to pick the worst then maybe Vex 4: Jhin is unplayable unless you get the augment that stops time or sniper's nest 5: Viktor