Hm. You asked for the worst kingdom, which means 10 cards and their interactions, but give reasons for single cards to be included.
So I'll try and mention some awful themes of kingdoms or at least 2+-card-combos.
Number 1:
"This is a singleplayer game": KC-Goons-Masquerade pin in some variation
Slight variation: Number 1a: "I'll play for both of us": KC-Possession madness
Number 2:
"The game ends when I buy everything":
I just don't find the log. Edit: Found the log. Anyway, you probably all know the sologame approach where Geronimoo bought all cards available within 12 turns. Nice because it introduces the 20-pile-ending as well.
Number 3:
"What do you mean, you can't buy anything?": This needs some preparation. Explain first how trashing is key in most games, and that you want to get rid of your starting cards as soon as possible. Then propose a game with Mint, Chapel and no useful 2$ cards + maybe Sea Hag or IGG. Your opponent hopefully opens Mint, maybe even Mint/Chapel, and you just add some Curses to his deck as well.
Number 4:
"What do you mean, you can't shuffle this anymore?": Your opponent probably won't value Ambassador, so no boring tennis games should be expected. But, with ambassador, KC, one card to draw them all and many mostly useless cards, you can get to an engine first and afterwards pump his deck with Curses, Coppers, Potions, Scouts, Talismans, what you can think of. Make sure to leave the last card in most piles so the fun doesn't end early.