The best gem choice for any genre is 1) support the author and 2) top-up points.
Those two are the only ones I’ve chosen if i had gems. Most especially STA ones.
I think Mystery, when solving a case, avoid gel choices to help readers get clues. I think it would take the fun out of solving the case.
In Romance and Drama, other characters’ POV and bonus scenes for specific non-canon ships would be nice, but won’t be the top choice if it’s not that big of a ship.