It really depends. If you just want something you can learn in a second; look at their chest. It's right in between their arms, and shows what they're going to do.
If you want to become as good as possible, look at their eyes. If you tried to do it without any training, it's very easy for them to lie with their eyes (for example, looking at your abs and then punching you in the face, or looking tired and then bursting at you). It takes a lot (a LOT) of training to learn what they're thinking, but you'll get slightly better at it, sparring session by sparring session, day by day, week by week, month by month, year by year, until you become good at it. When you do that, you can reach a peak that's MUCH better than looking at their chest.
When Hitting them focus more on the face area and chest to be more accurate when youre blocking or theyre throwing punches focus more on their foot movement and gloves to dodge the shots
Looking at the chest gives you visibility of their shoulders and feet. So look at the chest.
Look at their nards, so you can kick it?
I've herd that you should look into your opponents eyes and I've also herd you should look at their chest or their gloves, what do you think?
