I now know which play is the worst of them all.
Usually Titus Andronicus is the one that gets slagged off as the worst, but I strongly disagree. After seeing Julie Taymor's film and the BBC television version, it's clear that the play is a sensibly constructed tragedy, albeit a tragedy saddled with an over-the-top conclusion. I can still remember lots of good lines from it: "Rome is but a wilderness of tigers"; "Here grow no damned drugs, here are no storms, / No noise, but silence and eternal sleep" ; "If there were reason for these miseries, / Then into limits could I bind my woes."; and so forth.
No, no, the worst play is clearly The Two Gentlemen of Verona. There are no memorable lines at all, and I only found two passages that were vaguely worth quoting. The conclusion is completely ridiculous, even for a comedy; Proteus threatens to rape Silvia but is stopped by his friend Valentine who is secretly watching, after which Proteus's former love Julia upbraids him, Proteus is very sorry, and everything is forgiven in about 60 lines. It's among the earlier of Shakespeare's plays, but it was written after better plays such as Titus and just before Romeo and Juliet, so Shakespeare wasn't a neophyte playwright.
On our trip to Atlanta, Barb and I attended a performance of Two Gentlemen. I seem to recall we thought it was mildly amusing but not very memorable (except for the world-weary mastiff who played the part of Crab, a dog belonging to one of the comic servants). On reading, though, the play is dire.