In my opinion, the best drill for improving reaction time is practicing with a wall. Hold your tennis racket in your hand and stand about 5 meters from a solid wall. Throw a tennis ball onto the wall and hit it with your racket as it comes back. Your aim is to keep hitting the ball as long as you can while trying to keep it from hitting the ground.
Once you start getting better, you can vary your racket speed, change reactions and stay farther away from the wall to make it more challenging.
Here’s a post from BlazePod Answers, it can help you.
It has a number of reflex and reaction time improvement drills that can be helpful for getting better at agility, speed, and reaction time