1. Detective Conan: Just for being this good, this intriguing, this consistent for this long without almost ever boring me is all the explanation I need (and any other explanation is just explaining what you already know).
2. Haruhi: Unbelieveable writing for both comedy and drama. It's like a love letter for anime with both an insanely well crafted story and parodies all combined into something amazing.
3. One Piece: The new Dragonball, it does fighting shonen better than anything. The adventures, the worlds, the characters, all so distinct and fun and memorable. And it always seems to improve and always seems to move forward.
4. Welcome to the NHK!: You will never find such real characters in anime, I guarantee it. If you've ever had social problems or even known anyone who has, it will make you laugh, then cry, then both at the same time.
5. Depends...but I'll say Naruto. Just for keeping me interested since 2005 and for truly creating this world you will never mix up with the world of any other anime. It's pretty much the FFX of anime. You either love it or hate it, but regardless, you probably remember it well.