And here is a bash script create based on @error2007s's answer. This script requires your aws profile and bucket name as variables, and downloads the latest object to your ~/Downloads folder:
#!/bin/sh
PROFILE=your_profile
BUCKET=your_bucket
OBJECT="$(aws s3 ls --profile $PROFILE $BUCKET --recursive | sort | tail -n 1 | awk '{print $4}')"
aws s3 cp s3://$BUCKET/$OBJECT ~/Downloads/$OBJECT --profile $PROFILE
