Gladiator's plot is ridiculous.
It's hard to name a non-documentary film where the plot isn't ridiculous.
Since you resurrected this thread, I was thinking that the number of films that have aged well is a lot shorter and easier to remember than the ones that seem awful upon viewing today.
Even threads like "100 Top Westerns" really only have 10-15 movies tops that are actually worth sitting through. Westerns aged poorly for a whole host of reasons that teen sex/angst movies didn't (racist crap vs Indians or Mexicans or both, John Wayne bad acting, good guys don't miss a shot while bad guys miss them all, etc. etc.)
Anyway, looking back over what was mentioned and what wasn't the first time around on this thread, the original "The In-Laws" is not on TV enough and never gets the credit it deserves as a timeless comedy.