I think there are different definitions for "worst"
There are games that are just designed so poorly that they are nearly unplayable, then there are games that play fine, but are just really bad games. There is also the very subjective "games one personally can't stand", which others could love. An example of this for me would be Windwaker. I can't believe how much that game bothered me/let me down - yet others absolutely adore it.
From a technical standpoint, one of the worst games I can think of that I have played is "Star Wars: Masters of Teras Kasi". The gameplay was awful and controls are some of the worst the PS1 has to offer. With all of the great fighters on PS1 (Tekken, MK, SF, Soul Blade, etc) you'd think copying that formula and adding Darth Vader would have been a no brainer...nope.
From a "wow, this game sucks" perspective, I would say South Park on PS1. The game was so repetitive and boring, it got old in about 2 minutes after pressing start. And was just a really disappointing game, especially when compared to the N64 version - which was nothing special, but mils ahead of he PS1 version.
And just to continue the PSX trend, a game that I was personally very let down by and consider one of the "worst" is WWF Smackdown. After the success and awesomeness of Warzone and Attitude, Smackdown! was just a disappointment both in the creative department as well as story telling.