Squatting! Although any and all upright positions, along with following your bodys ques will give you the best possible outcome. There have been some great ideas shared here so i wont go into more details on those, but squatting is extremly helpful. 1.) it expands the pelvic outlet by 10% 2.) it puts the baby in a more optimal position 3.) gravity can aid the baby out 4.) bc your tummy is kind of tucked, mama can push more efficiently and can help "bare" down on the baby to push baby out.
I personally used a variety of positions with both of my births. Ive done kneeling, all fours, squatting, side laying, etc. But the most effective for me was squatting with resistance. That is ultimatly the position i was in when the baby was actually born (for both of my boy's deliveries), but i did utilize various positions while actively pushing. I successfully birthed one face up baby (and they are notorious for causing issues getting out) and one shoulder dyscotia baby (also notorious for the same). I can honestly say doing what my body told me to and utlizing mentioned positions helped achieve those vaginal births without any assistance.
I agree with the ladies about all the positions they mentioned though. They are all excellent choices. Follow your instincts and you'll be in good hands.