There are surely other Valentine episodes I've never seen, and probably some I just don't remember. This will just be things I either remember, or else probably saw, since I watched the series in question. Oh, and while the holiday may play a particularly prominent role in many of the episodes listed here, sometimes it's only a very minor aspect of the episode's plot.

Kim Possible
Season 4: The Cupid Effect (Amazon; IMDb; OHS Wiki; TV Tropes; Wikia)

Love & War
Season 1: Valentine's Day (Amazon; IMDb)

Mad About You
Season 1: Love Among the Tiles (IMDb)
Season 2: Cold Feet (IMDb)
Season 7: Valentine's Day (IMDb)

Major Dad
Season 2: Valentine's Day (Hulu; IMDb)

Miraculous: Tales of Ladybug & Cat Noir
Season 1: Dark Cupid (IMDb; TV Tropes; Wikia)

Mixed-ish
Season 1: This Charming Man (Amazon; Hulu; IMDb; Wikia)

Modern Family
Season 1: My Funky Valentine (Amazon; A.V. Club; IMDb; OHS Wiki; Wikia; Wikipedia)
Season 2: Bixby's Back (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
Season 4: Heart Broken (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
Season 6: Valentine's Day 4: Twisted Sister (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
Season 8: Do You Believe in Magic (Amazon; A.V. Club; IMDb; Wikia; Wikipedia)
Season 9: Written in the Stars (Amazon; A.V. Club; IMDb; Wikia)
Season 11: Paris (Amazon; A.V. Club; IMDb; Wikia)

My Little Pony: Friendship Is Magic
Season 2: Hearts and Hooves Day (Amazon; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia) Set on a Valentine's-like holiday.
Season 8: The Break Up Break Down (Amazon; IMDb; OHS Wiki; TV Tropes; Wikia)

Ned's Declassified School Survival Guide
Season 2: Valentine's Day (Amazon; IMDb; Wikia)

New Girl
Season 1: Valentine's Day (Amazon; A.V. Club; IMDb; Wikia)
Season 4: The Crawl (Amazon; A.V. Club; IMDb; Wikia)
Season 6: Operation: Bobcat (Amazon; A.V. Club; IMDb; Wikia)

The O.C.
Season 1: The Heartbreak (Amazon; IMDb; Wikia)
Season 2: The Lonely Hearts Club (Amazon; IMDb; Wikia)
Season 3: The Heavy Lifting (Amazon; IMDb; Wikia)
Season 4: The Case of the Franks (Amazon; IMDb; Wikia)

Picket Fences
Season 1: Be My Valentine (Amazon; IMDb)
Season 2: Supreme Courting (IMDb)

The Pretender
Season 2: Gigolo Jarod (Amazon; IMDb; Pretender Centre; Wikia)

Pucca
Season 1: Evil Love/A Better Boyfriend/Prince Not So Charming (Amazon; Wikia)

The Real O'Neals
Season 2: The Real Heartbreak (Amazon; IMDb; Wikia)

Rock Lee & His Ninja Pals
Season 1: The Battlefield Called Valentine's Day! (Hulu; IMDb; Wikia)

Sabrina the Teenage Witch
Season 1: First Kiss (Amazon; CBS; Hulu; IMDb; Wikia)

The Simpsons
Season 4: I Love Lisa (Amazon; A.V. Club; IMDb; OHS Wiki; TV Tropes; Wikia; Wikipedia)
(There have been a few other Valentine's episodes of this show, but this is the only one I'm sure I've seen.)

Single Parents
Season 1: A Cash-Grab Cooked Up By the Crepe Paper Industry (Amazon; Hulu; IMDb; OHS Wiki; Wikia)
Season 2: Chez Second Grade (Amazon; Hulu; IMDb; Wikia)

Smallville
Season 6: Crimson (Amazon; IMDb; TV Tropes; Wikia)
Season 9: Persuasion (Amazon; IMDb; TV Tropes; Wikia)

South Park
Season 1: Tom's Rhinoplasty (Amazon; A.V. Club; Hulu; IMDb; TV Tropes; Wikia; Wikipedia) Emetophobia warning. Also spoiler alert for "Contact."

The Spectacular Spider-Man
Season 2: Gangland (IMDb; TV Tropes)

Speechless
Season 1: V-a-l-- Valentine's D-a-- Day (Amazon; Hulu; IMDb; Wikia)
Season 3: J-i-- Jimmy V-a-l-- Valentine (Amazon; Hulu; IMDb; OHS; Wikia)

Suburgatory
Season 2: Blowtox and Burlap (Amazon; A.V. Club; IMDb; Wikia)

Supernatural
Season 5: My Bloody Valentine (Amazon; A.V. Club; IMDb; OHS Wiki; TV Tropes; Wikia)

The Troop
Season 1: Speed (Amazon; IMDb; Wikia)

Tru Calling
Season 1: Valentine (IMDb)

What I Like About You
Season 1: Valentine's Day (Amazon; IMDb)
Season 3: Stupid Cupid (Amazon; IMDb)

The Wonder Years (1988)
Season 3: The St. Valentine's Day Massacre (IMDb; OHS Wiki; TV Tropes)

The Wonder Years (2021)
Season 1: The Valentine's Day Dance (Amazon; Hulu; IMDb; Wikia)