Anything involving time travel. It's the sci-fi equivilent of jumping the shark. There needs to be a viewer warning at the beginning of such episodes stating:
Warning!
Our writers are currently out of good ideas. So, we threw this lazy shit together, which is going to be completely unsatisfying and will leave you with a vague feeling that the show should just end and let the writers move on to something new.
Viewer discretion is advised.
As an added warning, any episode which involves going back to the real present day should end the above warning with 20 minutes of Bobcat Goldwaith screaming.
I thought DS9 had some great time travel episodes tbh. Past Tense and The Visitor I think are top tier episodes, plus some fun antics between Far Beyond Stars (if that counts as time travel) and Trials and Tribble-ations.