As a very defensive player myself, I would say there are really 5 types of strategies that help break a good defense. I'll try to keep them all relatively short, but if there are any you feel you need more specific examples on, let me know.
#1 - Under Pressure: This method of breaking through defenses requires consistent, SAFE pressure on your opponent. Remember that all defensive options get worse the more someone uses them (shields get smaller, rolls get stale, etc.) Even if you aren't getting in damage, if you play a character that can safely throw out hit boxes and force your opponent to react without opening yourself up too much on a whiff or block, this is a viable option as it forces them to be more aggressive or slowly be whittled away. This seems to be most effective with Swordies, who have a good combination of frame data and disjoints to threaten safely.
#2 - Hook, Line, and Sinker: Rather than chipping away at an opponent's defenses, this method relies on baiting your opponent into specific actions by making yourself appear more vulnerable than you actually are. However, when they go for an "opening", they're really just walking into your trap. While an opponent may wise up eventually, you can get some serious damage in before they do. The best characters at this strategy are those that have some options with deceptive amounts of ending lag, or those with good quick burst options.
#3 - You Learned Wrong: Conditioning is the art of teaching your opponent to react to certain situations in certain ways, then punishing them for doing so predictably. If you consistently do something like Fair -> Grab, and your opponent always responds with shield -> spot dodge, that's awesome. Now you should try replacing that grab with a slightly charged smash attack, and END THEM. Characters like heavies who have more powerful punish options tend to be better at this.
#4 - On the Back Foot: An important skill for anyone trying to beat defensive players is to keep them in disadvantage as much as possible. When you do land that stray hit, be ready to capitalize on it. If there defensive neutral game is a problem, do everything in your power to stop them from returning to the neutral. Even if it means giving up a little damage, it may be worth it to keep them unsafe and in a bad position to defend. Rushdown characters are awesome at this, since they can stay in someone's face and rack up a lot of damage.
#5 - Fight Fire With Fire: You know what really defensive players are pretty bad at? Dealing damage when an opponent won't open up. If you have the lead on stocks or percentage or score or whatever, you have no obligation to engage, since the timer is now your friend. Sure, throw out some safe moves, poke with projectiles, keep them on their toes, but at the end of the day they are now the one that needs to hit you, not the other way around. Zoners and other good campers excel at this, and I don't advise using it to try and beat a defensive opponent unless you are also good on defense yourself.
Hope this helps!