If I personally decided to choose a new waterproof jacket today I would start from the bottom! I would ensure it was long enough to cover my bottom, (some are just too short) to have a two-way zip, the sleeves to reach to the end of my middle finger when arms are hanging down, I prefer a strap secure around the wrist, not elasticated, to have one or two chest pockets (lower pockets can often be unusable when the rucksack is strapped across them) and to have an underarm ventilation system as well.
I would give the hood special attention, some are better than others. I prefer a 'wired' one. I would look at how well the hood secures around my face (can be very important in really wet and windy weather) give special attention to how it fastens under and over my chin.
As to its waterproofing qualities, I'm not sure? When I bought my first jacket, a Paramo Alta 2, it was fine as regards length, sleeve length, strap security around the wrist and the quality of the hood but what I didn't like was it was heavy (just my opinion) made me far too hot in the summer months, did not pack away small and when it rained I was never really sure if I felt wet inside because of the rain outside, or if it was just condensation wetness caused by my body heat?
I then bought a Marmot Precip, a single skin jacket, much lighter, and I could roll up smaller. The downside for me was the elasticated wrists and still the problem of condensation on the inside.
I hope I have contributed something useful?