I prefer flat backplane with contoured keys.
A few years ago I posted a thread asking the same question, but with more options: Curved backplane with angled keys, flat backplane with contoured keys, flat backplane with staircase keys, flat and chiclet.
Curved backplane with staircase keys won the vote. Back then Cherry MX keyboards were not as dominant as today, and I got the impression that most voters used the IBM Model M, which has this profile.
Flat backplane with contoured keys got second place.