When I switched to mirrorless in 2012 I chose the micro 4/3 system with the Panasonic GH2. On of the most offered lens of the system was the Panasonic 20mm F1.7. Although it had some shortcomings like the noisy and slow autofocus motor you could hardly complain about its image quality. It was sharp straight from F1.7, one of the best amongst the micro 4/3 lens. I used this lens a lot on photo walks and holidays. This was the point when I fell in love with the equivalent 40mm focal length.
Full frame 35mm is thought to be the most versatile focal length. When I go on a day trip with my family most of the time I use this. So far my old Canon nFD 35mm F2.8 did the job for me. Although being not a recent design it's a very solid performer on the Sony A7. The only thing I really missed was the autofocus capability. When the first rumours emerged that Samyang was going to release a native, 35mm, AF, pancake, full frame lens for Sony E-mount knew I would go for it.