there's three periods of anime for me. those that were out while I was in the anime club in college (by 2000, before i had money or a dvd player), those that came out after I started watching most of my anime online (after 2008), and those that came in between (found out about them at anime conventions, NewType USA, Netflix DVD rentals, or buying the DVDs). i prefer not to compare titles that come from different "eras".
of the older titles, Ranma 1/2 is king. of the newer titles, Fairy Tail reigns supreme. of the in between titles, it'd be Azumanga Daioh.
other early favorites: Slayers, Tenchi Muyo, Ah My Goddess, Battle Athletes, You're Under Arrest, El Hazard, Bubblegum Crisis, Iria, Dragonball Z, Sailor Moon, Armitage III, Revolutionary Girl Utena, Rurouni Kenshin, Saber Marionette J, Lodoss Wars
other middle favorites: Chobits, Read or Die, Scrapped Princess, presa Panic, Real Bout High School, Vampire Hunter D Bloodlust, Hellsing, Full Metal Panic, High School Rumble, Samurai 7, Fruits Basket, Ninja Scroll the Series, Jubei-Chan, Angelic Layer, Inu Yasha, Melancholy..., Death Note
other newer favorites: Wagnaria, Hayate the Combat Butler, Koihime Musou, Soul Eater, Modern Magic Made Simple, The World Only God Knows, Birdy the Mighty Decode, Charger Girl Juden Chan, Powerpuff Girls Z, Rosario + Vampire, Squid Girl, Tiger and Bunny