tl;dr - Make posts about Spacing, Returning to the Stage From the Ledge, Retreating, Killing, and Defending.
For everything: Link is versitile so mix up your options.
Spacing:
Link has slow frame data but good disjointed attacks. As a rule of thumb try to keep the opponent no closer than the tip of Link's attack. The opponent will try to get in so aiming to hit with the tip of Link's attacks leads to good spacing. Some MUs might have you spacing farther.
Returning From the Ledge:
IMO the quick stand up doesn't help Link as much as other characters due to his slow QCQ. Link doesn't have any perfect ways to climb back on stage but he has a lot of mix ups.
Ledge hops (tap away from the ledge>tap back toward the stage+jump>enter an attack) can be good if the opponent is far enough away but if they hit you you'll be offstage without a 2nd jump. Fair, Nair, Zair, boomerang and arrows all work well from a ledge hop depending on the distance. If you're not being threatened you can ledge hop bomb pull to save time.
Ledge jump is good if the opponent is hanging back on the ground. Bombpulling from a ledge jump gives Link a trump card.
Ledge rolls are best if the opponent uses an aerial to hop offstage or does a long lasting move at the edge.
Ledge attacks aren't great but can be a quick, safer than ledge hop, attack to punish the opponent if they commit to an action.
Mix up recoveries to not be punished.
Retreating:
Zair/Fair from a backwards SH can stop opponents trying to rush Link. Zair with a bomb from a SH will C4 it and make it harder for the opponent to approach. On the ground pivot Ftilt/Fsmash works very well.
If you're a bit farther SH/Jump and throw the boomerang down or pull a bomb.
Platforms are Link's friends (except maybe the SV one that Link can't jump to). Link has good ways to attack below him with bombs and low landing lag so platform camping is often more effective that moving back on a flat surface.
To escape pressure and reset a situation:
Jump/SH Nair, jabs and rolls. Jab 2 cancel>roll away.
Killing:
If the opponent approaches Link you can use defensive options like pivot attacks, punish grabs etc. If Link has to hunt the opponent down it's much harder to kill.
Fake outs: SH Fair can hop over projectiles and attacks and hit with the second swing. Rush>jump>throw bomb down>aerial or footstool lock. Throw bomb and if the opponent has a reflector>dash attack to catch it. Also bomb throw>grab the opponent's shield.
Chains/locks: Jab 2 cancels put Link in a position to read the opponent. Zair/Nair>Land>Dsmash/upB/jab. Bair>land Utilt or upB. Throw bomb down at opponent>footstool>FF Nair>Fsmash or arrow for a lock. Bomb throw>SH Fair/Uair or grab if the opponent shields.
From Grabs:
Grab>Dthrow>buffered dash>Jump>Uair kills at medium-high %s. Uthrow kills after 160%-180% depending on the opponent and Link's rage; sometimes even lower with low ceilings.
Punishes:
Dair kills off the top at high %s like 140% ish and lasts 50 frames- longer than any dodge. Uair outlasts air dodges and punishes platform landings. Dash attack punishes rolls and landings. UpB punishes spot dodges or if the opponent rolls into you. Ftilt punishes ledge hops/stalling and some SHs.
The fastest, safest kill move Link has is Utilt. It can work in most situations. U/Dsmash and UpB start fast but have a lot of endlag if you miss. Fsmash is Link's strongest kill move from a read, it can punish dodges with Fsmash 2 but is very punishable on shields.
Link can also go offstage and intercept the opponent or arrow snipe them.
Defending:
Generally good spacing is the best defense. Platform camping, projectiles, playing passive-aggressively. OoS Link can roll, SH/Jump Nair but it's better that he doesn't get caught in his shield. Jabs are good for stopping rushdown opponents but smashes generally are too unsafe if they wiff. All the retreating stuff could fit here.