Every (most?) classic film fans will, at some stage or another, find themselves wishing they could see a film considered lost. Common ones include Tod Browning's "London After Midnight" starring Lon Chaney or Murnau's "4 Devils", both available in reconstructed approximations using stills - most lost films don't even have that. Others like "Sadie Thompson" with Gloria Swanson survive in trucated forms, in this particular case nearly complete, but on the other hand only one reel has been found of "The Divine Woman" with Garbo (there are other instances where only small fragments survive). Occasionally, miracles happen - the Gloria Swanson/Valentino only pairing "Beyond the Rocks" was found in the early 2000s and more recently, the nearly complete director's cut of "Metropolis".
And of course there are versions of films which were never released, like Orson Welles' cut of "The Magnificient Ambersons" which I think would top anyone's most wanted list. Although even in this department miracles do happen, as the 2004 finding of the pre-release cut of "Baby Face" now available on the "Forbidden Hollywood vol 1" DVD set (not holding my breath for "Ambersons" though).
So, which one is mine? Well, it's an infamous WB Pre-code comedy called (yes, you've guessed it...) "Convention City" starring Joan Blondell, Mary Astor, Adolph Menjou and an array of other familiar faces of the studio. This is possibly the last major production of a Hollywood studio to be lost. After the Hays Code was fully enforced it remained mostly in the vaults (although it has been shown somewhere in the US in 1937). It has since gained a reputation of being too daring even within Pre-Code limits (I doubt it can be "worst" than "Baby Face"...). A full background can be found in this thread here. Because it did had a proper release there is hope it one day may resurface, even if it's in a truncated copy. My fingers are crossed. Yet, do I really believe that it will live to my expections? Honestly, no. It was probably as memorable or as forgetable as most of the comedies produced by WB between 1930 and 1934 - but I certainly wish I had the chance to see it.