On the older consoles I never owned anything I really hated, maybe because I was a dumb kid or I just got really lucky with my choices.
That said, on more recent generations, my family hasnt been as lucky with the choices. There are a range of average games in our (especially PS2) collection, and a few terrible ones, to choose one will be hard but heres a few of them.
AFL Live 2004 - Terrible graphics right from the start, even for the time.. I swear there are about 5 character models, with every model looking exactly the same with the exception of skin colour and hair colour. When the footy is kicked, you can see even in the action that it doesnt make contact with the foot, in fact it misses by about 3 feet. But who cares as long as the gameplay is fun?.. The first issue is the difficulty, there is no middle ground, its either so easy that if a goal is scored against you your letting it happen on purpose.. Or its so hard that every time you touch the ball your tackled, the computer WILL mark the ball and is capable of superhuman 70 metre kicks on a regular basis.
The game just had nothing to play for, the gameplay is very simplistic and maybe you could get one or two games with a friend (presuming those games are very short), but then its all just the same from there on. There is no benefit from playing Geelong over Port Adelaide, they are all exactly the same, only with different player names (which the commentators get wrong over and over again). Its just a bad game with no reason to come back to it.. Actually no, its one of those games where you leave it, then come back to it in a few months wondering why you thought it was so bad and then you remember all over again. The worst kind of bad game.
Smarties Meltdown - Played it once, had a seizure.
Sean Palmers Pro Snowboarding - Like Tony Hawks except you can only go downhill (understandably), the levels and characters are all bland, and while realistic.. Only being able to go one way gets boring fast, and it also means you have to redo levels more.
Boogie - Came with the console, so I can at least have some dignity knowing I didnt buy it.. A game where you sing pop songs on mic while dancing using the wii mote and nunchuck. So the first issue is holding three things at once, the dancing consists of about 4 moves, some of the songs are actually good, but the game is just so pointless and not fun. The fact that I played it a few times destroys that dignity I had instantly.
Monopoly Party - Like the board game, without the fun (or the cheating).
With so many games in that collection, its hard to pick out the really terrible ones. Some of them are just bland (Enter the Matrix), some of them have just pissed me off so much but arent necessarily bad (Dropship United Peace Force), some of them have got things right, and then have issues that majorly annoy me (Just Bring It)
I think the worst game Ive ever bought in my mind right now is AFL Live 2004, just because I was hoping it could be so much fun, a good AFL game... I tried to like it, I couldnt, I once did a premiership with friends, they all chose teams, I chose Port Adelaide, and didnt even get up to my first match because the game was turned off due to frustration and boredom.