Studying defence is the most important part about learning boxing, most of the time at the start of your journey you wouldn't be the one that always attacks so having good defense is crucial to develop further as a boxer.
Here's a list of boxers with amazing defense worth to study:
Saul Canelo alvarez - head movement master , one the best mexican boxers off all time
2. Floyd Maywether - philly shell , master of the ring
3. Roberto Duran - one of the 4 kings of boxing , master of smooth parrying and defense
4. Niccolino Loche - "The Untochuable"