"Once More With Feeling" (Buffy) - I know every word of every song (except for "Dawn's Ballet," which is, in fact, instrumental).
"The Gift" (Buffy) - The Slayer dies to save the world. You don't get much more epic than that.
"Touched" (Buffy) - In a dark moment, during a dark, dark season, Spike really shines.
"Not Pictured" (Veronica Mars) - The fantastic season 2 finale solves two mysteries with one huge twist. And Logan saves the day.
"Spit & Eggs" (Veronica Mars) - Once again, mystery solved, Logan is valiant, tears are shed.
"I Do" (Lost) - Nathan Fillion guest stars AND Kate and Sawyer have cage sex. Best episode ever?
"Through the Looking Glass" (Lost) - A legendary finale, and a teary but heroic farewell to a beloved character.
"Happily Ever After" (Lost) - Double the Desmond, double the fun.
"Hidden" (Smallville) - Clark has to save the day, sans super powers. (SPOILER ALERT: It's awesome.)
"Lovesick Ass" (Wonderfalls) - Jaye and Eric have to babysit a deranged little boy and his Russian mail-order bride. I kid you not.
"Kerplunk" (Pushing Daisies) - An amazing finale to an amazing show, canceled too soon.
"War Stories" (Firefly) - Mal and Wash on a mission together, two of the greatest characters in the history of television. Also, Mal loses part of his ear (but reattaches it).
"Stop-Loss" (Dollhouse) - I loved this Victor-centric episode. Someone give Enver Gjokaj his own show, stat!
"A Stone of the Heart" (The Black Donnellys) - Odds are you haven't even heard of this show about brothers trying to hold their own against local crime lords, but it was phenomenal, and this episode is the embodiment of why.
"The Parts in the Sum of the Whole" (Bones) - The last three minutes of this episode make the first five and a half seasons worth it.
"Chuck Versus the Other Guy" (Chuck) - The culmination of three seasons of awesomeness.
"Wind Sprints" (Friday Night Lights) - Between Tim Riggins trudging through the rain and Lyla's heart-wrenching breakdown, this episode clinched the series for me.
"Friday Night" (Joan of Arcadia) - I can't decide if this episode is too sad, or just sad enough to be my favorite episode of Joan. Jury's still out.
"Raincoats and Recipes" (Gilmore Girls) - Luke and Lorelai are interrupted by Kirk, who runs down the stairs having one of his night terrors...naked.
"The Best Chrismukkah Ever" (The O.C.) - Say what you will about The O.C., but it was a phenomenon, and it gave us the greatest bi-religious holiday ever.
"At World's End" and "The Wish-Pretzel" (Greek) - I'm counting these two episodes together as the pinnacle of the Casey/Cappie love saga.
"Yeah, Just Like Tomatoes" (Weeds) - Zooey Deschanel guest stars in this already-hilarious masterpiece of a show. Kat (Zooey): "I'm part of this family!" Nancy: "Just because you are crazy, and we are crazy doesn't mean we're related."
"Miracles" (How I Met Your Mother) - Barney and Ted finally make up. (Everyone knows their bromance is the real love story of this show.)
"Casino Night" (The Office) - Jim and Pam: The early years. I could watch this episode on repeat and still adore it.
"Mamma Mia" (30 Rock) - Tina Fey + ABBA + Alan Alda = hilarity
*ADDENDUM (by which I mean, essential episodes I somehow overlooked and have now edited this entry to include):
"I Will Remember You" (Angel) - For one episode, Angel and Buffy actually had the life together we all wanted for them.
"The Constant" (Lost) - I tried to limit myself to three Lost episodes, but who am I kidding, this Desmond-centric episode is one of the top episodes of primetime TV ever.
"Making a Stand" (Arrested Development) - How could I forget this hysterical show? This episode taught us that using your amputee buddy to traumatize your children is a great way to teach them life lessons. "And that's why you always leave a note."