Screen 1: Speed (large), heart rate, distance, total ascent, grade%
I chose these because that's what I want to know as I ride - I am usually trying to go a certain distance, I need to know if my heart rate is higher or lower than it feels in order to gauge effort, I like to amass ascent feet on some rides and guess how many feet a short rise is then confirm it, and I'm always interested to see what my grade % is.
Screen 2: Ride time (so I can tell my wife my estimate was close to reality - I'm actually very good at that even on unknown routes), average heart rate, average speed, calories and time of day. I want to look at average speed when I get home but not on the bike so I am not in a race with myself. Average heart rate is an interesting field as it indicates both effort and general level of exhaustion - it tends to go down on the latter days of multiple sequential long rides. I almost never look at screen two during a ride.