The best gift for boosting customer experience is subjective to the specific customer. For the gift to improve their experience with your company and have a long lasting effect, it should be an item they really like. When building these relationships, it is important to pay attention to what the customer says in terms of passions and preferences. If you get them a gift that aligns with either of those, there is a good chance that they will remember you and that gift for a while.
If you have a hard time learning what they like, try sending a gifting link which allows them to purchase an item of their choice based on a budget you set. This helps ensure they enjoy the gift and they will definitely be thankful for the gift of choice.