Mostly been using 2nd and third gear.
I use 2nd up to 25 mph or so and then 3rd above that.
Here's a video made a while back testing acceleration in a couple different gears - no shifting - just put it in gear and go.
This was pretty much flooring it, meaning I was using all amps I had.
My controller is a 400 amp peak, but my ammeter only goes up to 300, and I've never really had a way to check how much I really can pull out of the controller or not.
For example, there is an adjustment (by screwdriver) in the controller to limit current. I don't even know for sure if that is all the way up or not.
I do know that the ammeter goes up a lot faster in a higher gear while accelerating than in a lower gear.
EDIT: Link won't work right - here's the video directly.