Doesn't have to be your favorite anime :P cause i know some extremely good animes weren't really addicting.
I'll list a few that I couldn't stop watching.
Hunter x Hunter - I watched all of it in like. a week or something. Even though I finished watching this over a year ago I wish I could watch more O_O.
(
Off topic: The end of this anime is when they leave Greed Island right? Can someone clear this up for me? What is "Chimera Ant Story Arc"
http://en.wikipedia.org/wiki/Hunter_..._Ant_Story_Arc)
Naruto - I was addicted until probably around episode 170, I got really tired of it after that...
DBZ - If I watched it again I probably wouldn't be addicted but back when I was little I couldnt stop watching it!
Love Hina - Dunno, it was good
Hikaru no go - um, it was really good too.
Ok I really don't have reasons anymore, just a straight out list.
Eyeshield 21,
Whistle!,
Prince of Tennis. And some parts of
Bleach were addicting.
But, by far, Hunter x Hunter was the most addicting anime I've EVER seen.