For me Sport Mode (most of the time)
I have to caveat that because my/our other cars are an Barchetta and Strada Abarth 130TC. These are not slouches and throttle pedal response near on "let me go, go, go".
The 500X in Auto Mode will GO but in comparison to our other cars the pedal depression required is just way way too much travel required.
I drove my 500X in Auto Mode continuously for a couple of months and got used to it and was not disappointed. However every time I drove the other cars (during this period) I was finding I was launching to the moon.
Yes you can get plugin boxes to remap pedal response but this is what Fiat did with the Mood knob by design. They just got the mapping a little wrong IMHO.