Hard to come up with anything I really hate, but there's a long list of sequels that are worse.
For my very favorite games, Wing Commander 3(/4) was huge drop in gameplay from 1&2, I think Phantasy Star 2 is much overrated compared to the great original and - controversial opinion - I don't much like Monkey Island 2. Riven is the worst part of Myst series (but still not bad).
Perhaps biggest drop I can think of is Final Fantasy Legend, but that's unfair comparison because I played the first as fan translated Wonderswan version. Still, I though that was an awesome game and the second was just a generic JRPG, completely linear and by adding fourth class took away all interest in creating a team of four (first game has three character types; though actually in FFL2 you're better off using 2+2 of the better types).
Indiana Jones and the Infernal Machine is technically sequel to Fate of Atlantis, but different genre so that doesn't really count either.
Leisure Suit Larry 2 is worst of the series.
I enjoyed Star Wars: Battlefront (the old ones) much better than second. And prefer Battlefield 1942 to BF2.
Lego Racers. I wouldn't quite say I loved the first, but it's good. The sequel was awful.
Tex Murphy series: original Mean Streets was cool game, Martian Memorandum is bad.
Clock Tower (SNes) > Clock Tower (PS1, really CT2) > Clock Tower II (Ghost Head)
Dragon Warrior/Quest III is much worse than first two and DQ5 was disappointment after 4.







