I tried the Bebe's Kids ROM just to see what the "worst game of all time" was like... and yeah, pretty bad.
Worst game I've played that was not a ROM... either Teenage Mutant Ninja Turtles for NES or Monster Party for NES. Both games are extremely hard. But Monster Party for NES, I only got to the second level like once or twice in my life. You'd have to have cat-like responses and a lot of luck to get through that game, I tell ya. And I remember full well that sometimes it seemed like you couldn't dodge bosses as they moved towards you. It's like you needed to be Bert the flying gryphon thing during bosses to stand a chance. As for Teenage Mutant Ninja Turtles... do you remember at the end of like the first level for the second world (looked like an office complex or something) that you had to jump across a large gap or else fall down to the spot at the beginning of the level? It took me forever to finally jump that gap, had to jump at the last possible second. Actually, I remember you had to jump at the last possible second to clear the gap for the first level in Comix Zone too. Couldn't get past the second level of Comix Zone either, but at least it had an awesome style to it.
Actually, no, Dino Riki for the NES was the absolute worst game I've ever played. It wasn't fun, it was very cheap, never got past the first level. So it was slightly worse than Monster Party. Monster Party at least had a cool but creepy box cover. Dino Riki's cover was just terrible.
So, any game that I couldn't get past the first or second level on gets my vote for worst game of all time. I know there are people who appreciated this insane level of difficulty, but I can't believe in hindsight that the NES was home to such difficult games. How did people all over the world build up the nerves of steel to get through those games? Sheesh, no wonder we complain about lack of difficulty nowadays, because the difficulty is no longer at superhuman levels.
Oh, for the record, I might say sometimes that Snake Rattle N Roll was one of the worst games of all time, but it's not compared to my other choices. I got to the 5th level of Snake Rattle N Roll when I was a child, and managed to get to the 6th later on in life. Then with heavy use of save states, eventually beat all 10 levels. The gameplay was always your biggest enemy in that game. But I liked that game nonetheless.