The canonical forms written for formic acid (eq. 1) and methyl nitrite (eq. 4) are clearly different. The first structure in each case is a good representation of the corresponding molecule, and is the major contributor to the actual hybrid structure. The second structure in each case is a higher energy (less stable) form, due in large part to the charge separation. Consequently, resonance stabilization in these compounds is modest at best.
The remaining compounds and ion have two electronically and structurally equivalent canonical forms of low energy. Neither form corresponds to the actual structure of the molecule in question, which is best described as a resonance hybrid in which the two structures shown have equal weight. Thus the resonance stabilization of these molecules is substantial.