Hmmm, well the problem with choosing a worst game of all time is that I've played so many.
One big contender would probably be Future Wars:
http://en.wikipedia.org/wiki/Future_Wars
My big problem is that I played large numbers of truely terrible games on systems like the Commodore 64, and Apple IIe that they run together in my mind, and with the passage of time I can no longer keep them straight in my head or remember the names.
On the original NES I remember some of the movie tie ins like "Platoon" were thematically interesting, but a horrible mess to actually play. Then of course we had things like "Deadly Towers" which had some redeeming merits, but was sort of like the "Superman 64" or "ET" of it's day.
Then of course there is the Vectrex, which a guy I used to play PnP RPGs with when I was younger used to have. Difficulty aside, a lot of those games were fundementally broken in some small way once you got through them far enough. For example in it's version of Berserk I believe Otto could spawn pretty much right on top of your character, making it so dying could be almost entirely arbitrary, something I can understand in the arcade version (intended to munch quarters) but not so much for a home console.
Then of course you had the "Swordquest" games for the Atari 2600 which were released when I was VERY young. The only real redeeming merit to these games was that they were designed as a contest, and the obtuse and difficult gameplay was designed so that the "best" player could send it in to Atarti and win a prize. Basically there was a Talisman, a Sword, and a Crown that were created for the contest, and made out of precious metal and gems with a pretty substantial value. The contest was never finished, and the only item ever won was the Talisman which was apparently melted down for the metal and gems and sold by the victor (such a waste). Apparently the other two items still exist and are presumably the property of whomever runs Atari. To this day I've wanted them.
Oh and finally while I'm ranting, there is one game that to me is in a league of it's own due to being an awesome idea, but poor in implementation: The Star Saga games.
Star Saga One: Beyond The Boundary, and it's sequel were attempts to make computer based party games. The game came with a map, playing pieces, and all kinds of books similar to a choose your own adventure in their format. You'd go turn by turn with the computer, moving pieces around the table, and looking up book entries to find out what was happening. To be honest it was a neat idea, but for obvious reasons it didn't catch on. I'm actually a big enough nerd where as a kid I did it with my parents, and let's just say it wasn't what you'd call entertaining.
