All three
Law & Order shows, particularly D'Onofrio episodes of "Criminal Intent," the Moriarty era of the mothership show (years 1-4) and the Alex Cabbot era of "SVU" (years 2-5). I've seen all the episodes at least twice (some way more than others) and I could watch them a few dozen times more now that they've all been remastered in high-definition for cable syndication. It's like crack cocaine times 650+ episodes (and counting) plus my sister's addicted too.
Secret World of Alex Mack. Half the episodes I can't watch (too kiddie-oriented and dumb) but the other half makes me smile. In an era when the teens of "Gossip Girl" are considered cool its nice to see kid actors portraying relatively normal, troubled and decent teen characters with relatively normal problems about fitting in life and school. Especially if you have telekenitic/electric powers, can turn into water from a chemical spill and the town's bully CEO villain is out to get you.
Match Game reruns (1973-82) on Game Show Network. Shaggy carpeting, sexual innuendo up the whazoo, forgotten 70's celebrities and the unbeatable combo of Reilly/Sommers/Dawson/Rayburn as regulars make this old gameshow ridiculously rewatchable and hilarious.
Mystery Science Theater 3000. The older and wiser I get with age the more jokes and references I previously didn't get become LOL-worthy moments that had flown over my ignoramus mind in years prior.
What's My Line?, specifically the B&W kinescoped version GSN shows Monday through Sunday at 3AM. A time capsule of class and elegance put to the service of a simple panel guessing game, when being called an 'elite' was meant as a complement and suave emcee John Charles Daly presided over class acts like Arlene Francis and Bennett Cerf as they guessed average people's line of work. Celebrity Mystery Guest appearances remain thrilling 50+ years after the secret is out.
Honorable mentions: "The Incredible Hulk," "Sailor Moon," "Dexter," "The X-Files," "The Sopranos," "Cardcaptor Sakura" and "The Price Is Right."