It's my favorite anime, manga, and video game series so maybe I'm biased..
But a lot of people say it's one of those 'must watch' especially if you like depressing, psychological, friendship. I recommend the manga or sound novels; DEEN did bad with the anime (well it's not bad but it's confusing and a lot of stuff were taken out)
When They Cry - Higurashi, aka Higurashi No Naku Koro Ni.
I must warn it's not kids series though. It's a depressing psychological mystery-murder at first, then a depressing mystery. I recommend the manga or sound novel.