The most effective fighting stance for self-defense depends on the situation you are in and your own experience. The best stance for self-defense allows you to move quickly and easily, while also providing stability and balance. 

Use these stances for best results:

The Boxing stance:

You have to stand with your feet shoulder-width apart, with your dominant foot slightly ahead. Place your hands in front of your face, tuck in your elbows, (this will protect your body) and face your palms towards your opponent. In a real self defense, situation punching is not as good as palm striking because you can break your hand. This stance offers you mobility and enables quick punches and footwork.

The Muay Thai stance:

Adopt a stance similar to the boxing one, but instead, place your feet closer together. and not as long. Hold your hands in front of your face and flair your elbows slightly for protection. Put your hands higher than you would in a boxing stance to defend against head kicks. This stance provides stability and power, allowing for strong kick while keeping your legs further back to make it harder for them to kick you.

The grappling stance:

Use this stance low when you think they might try to tackle you . Stand with your feet hip-width apart and your dominant foot slightly forward. Hold your hands in front of your body, bend your elbows, and face your palms towards your opponent. This stance gives you great mobility and enables quick changes in position. However, this stance leaves you vulnerable to striking techniques, (like knee strikes to the face) so only use it as you take your opponent to the ground or defend against a takedown technique.

Here's the thing, when you take up a fighting stance or position you put yourself at a serious disadvantage.

First, it reveals to your opponent that you may know some kind of martial arts or that you have had some previous training.

Next, taking a fighting posture shows that you are willing, committed and prepared to fight. Consequently, the opponent is ready for the fight as well. Taking up a fighting posture may also lead to your opponent becoming even more aggressive.

The most effective fighting stance for self defense is the  surprise attack stance

In facing an aggressive adversary you should remain formless most of the time. Do not take up a typical fighting stance with your hands up like a boxer

Instead, when confronting a possible adversary, take a non-aggressive, normal, standing posture with the hands lightly resting on the front part of the hips. Keep in mind you are not just standing there, waiting to get punched, rather you are in a mode of readiness. You can move instantly and with full force from this posture.

This stance is the most common and practical of all the various stances for pure self defense. To assume the best stance for self-defense in the street, enter the ordinary boxing stance, with one big difference, your palms should be open, facing away from you. To visualize this stance think about someone would do naturally when saying "I don't want to fight" Under these circumstances, taking up a natural formless posture will permit you to use the element of surprise to its maximum advantage.

The element of surprise

If the fight is unavoidable, and you decide to move from this formless posture, it will be a complete surprise to the criminal or aggressor. The element of surprise is indeed a powerful fighting strategy. Your opponent will react differently when taken by surprise then if they were ready.

For example, when someone is expecting a force to come in contact with their body, they tend to naturally brace for the impact. This makes any strike to that braced area less effective.

Whenever a strike hits unexpectedly, it feels like it has twice the power.

You should definitely devote some of your training time to practicing self defense from the various sttance listed above. However, taking a fighting stance should only be done if the element of surprise has already been lost.

Ultimately, I want my students to understand that one should be ready for any situation.

All self-defense situations are inherently different. Imagine there is a thug / criminal / drug addict who is prowling about inside of your house. Subsequently, you come upon him in the middle of the night. In this situation remaining formless and trying to surprise the criminal would be useless.

The element of surprise has already been lost. This is a perfect time to take up a "fighting stance"

However, when someone being belligerent and trying to provoke a fight, for example, in a bar, this formless posture will allow you to use the element of surprise. The element of surprise is one of the kung fu practitioners' greatest weapons. This is one weapon whose power should never be underestimated.