Defining gialli is a tricky one 'cause it wasn't really used by actual Italians as a movie genre term when they were making them. It did refer to literature, but it was only really later critics (possibly Alan Jones was the main one) who made it a film "genre". Think the only film to directly refer to itself as such back in the day was Giallo A Venezia and there's a pretty high chance that the name was referring to how sleazy it was rather than it being an Argento/Martino/whoever imitant. This being said, I think of them as lurid, convoluted and improbable (in plot and/or psychology) mystery/crime films, with often an emphasis on aesthetics and the lurid over meaning or sense (not that they by any means all ignore deeper meaning or sense). And importantly for exclusion purposes, not straight supernatural (no Suspiria) or bodycount (i.e. slashers) or elaborate jokes (like Bay of Blood), but still descended from your Edgar Wallaces, Agatha Christies, Carter Dixons etc.

Of things others have mentioned I'll strongly second The Red Queen Kills Seven Times and What Have You Done To Solange. But no one has mentioned any that aren't good. Otherwise, The Girl Who Knew Too Much, Blood and Black Lace, The Strange Vice of Mrs. Wardh, and Your Vice Is A Locked Room And Only I Have The Key.