For a season to be the worst, there would be several categories for this but most of them are based on a season's cast, locations, design and suspense.
My nominations:
Season 8 - This was a horrible execution and I cannot even watch it. It was that bad and no leaving North America? The teams were mostly unlikable and annoying as hell and the NEL placement along with two KOR episodes, GOOD LORD! It is unwatchable and abysmal in every way.
Season 10 - I know many will probably disagree but I honestly can't stand this season. It's still watchable at least to some extent unlike the above but what ruins are the cast, the race planning and a heavily under utilized route. Here we have a season with so many unique locations you would think that it would have been one of the best in that department - sadly no. The Kuwait leg is underwhelming, and the same goes for Mauritius, Ukraine and especially Spain and France. I disliked all of the teams that made it far except the Cho Brothers.
Season 13 - I have only seen it once and didn't even finish past leg 9. Most of everything about TAR 13 is bad for me with a poor cast, terrible locations except Bolivia, New Zealand and Kazakhstan and just feels very cheap when compared to other seasons. I just don't like it, and it was released during the writer's strike which made it worse.
Season 16 - This one suffers from another genuinely unlikable cast except maybe the cowboys and a few others whom I cannot remember now. The route is fine but the cast made the race so hard to watch! It is also the season with my least favourite team of all time - Carol & Brandy. Too much ugliness and negativity throughout. The race was not exciting at all either.
Season 24 - The last entry on this list is a season many consider to be the worst and for the most part, I agree. However TAR 8 takes the cake for being dead last so this one has avoided the bottom position only just. This season has to be the most poorly planned experience in the history of the race, for we have teams returning for the third team, a 3 continent season containing multiple double legs and some of the worst task design I have seen. I did not enjoy a single leg of this crap.
Dishonorable Mentions: 4, 15, 19, 25 and 27