The 35/2.8 is an early (1951) design and very sensible for backlite flare, but resolution and colors elsewhere are excellent. contrast is on the lower side. The Nikon 35/2.5 shows much higher contrast here.
The 35/1.8 is a bit overstretched for speed. It was (together with the Nikon) the highest speed 35mm lens in the world back in 1957. It acts on the same niveau as the 35/2.8 when closed down a little. Contrast is better.
The 35/2 is the last, and best overall use lens. But contrast wide open is a bit pushed too far, and boke and color rendition wide open a bit ugly. With B&W no problems. The best lens at f/5.6-8, up to modern standards except for the Gaussian wideangle typical backlite flare (can be seen at the 1979' Summicron in the same amount)
The 35/1.5 is the sharpest Canon 35mm at f/2.8, and gives the best color rendition, IMHO. It's overstretched wide open too (again, it was the fastest 35mm in the world back than). With sidelite it shows unpredictable ghost pictures at most apertures even with a hood, but worse with none. But backlite flare i.e. tree twigs on a bright sky are a bit better than with the 35/1.8. It's surely not only faster but better corrected. It has a bit a bad reputation while it was too cheap. Probably it's as "bad" as the first-gen. Summilux 35/1.4 which users be calm about bad results for sake of resale value. Canon users typically are not as clever. Obviously the bigger glass of the Canon 35/1.5 will result in less light falloff wide open than with the Summilux, at the price of ghost pictures and coma wide open. the same goes for the 50/0.95 when compared to the Noct. 50/1. Leica decided the easier way to cut off glass size and edge light rays, allowing more light falloff with mostly B&W usage of their lenses in mind. With color film, probably more in mind of Canon designer JIRO MUKAI (all the lenses except the 35/2.8), light falloff is less tolerable. Sharpness at f/1.5 is just in the very center. Leave it wide open only if you don't get the picture otherwise or when taking cute kids/cats. But then, be sure have them placed in the center of the picture. After all a 35mm at 1m minimum distance usually is a bit too wide for cats portraits...