Two questions are enough when choosing a raincoat: how hard is it raining, and how long is the walk?
Short walk, sudden shower
A light shell that covers the back and chest will do the job. It fits in a bag and goes on in a few seconds.
Long walk, heavy rain
A raincoat with a closed belly that covers all four legs saves you a bath when you get home. Taped seams and a hood make the difference here.
Care
Fabric softener reduces water resistance; wash raincoats without softener and hang them to dry.