CLANNAD is the way to go.
CLANNAD was my first Key-anime, or rather, anime in general, that I watched. I think CLANNAD had some of the greatest values that a story could have. Friendship, and family-issues were the things that stuck to me the most. It’s beautiful to see a story filled with characters that you love, with a beautiful story.
I especially loved Tomoyo’s Route/Takafumi’s route for pointing out the beauty of family. Family that are blood related, or family that are friends. How you can overcome obstacles as long as you have love, such as Takafumi, how he jumped off a bridge and Tomoyo’s family became closer
And Kotomi’s parents’ words that point out that Life Is Beautiful, and how you should live it the way you want to, to the fullest. It’s sad when you realized that they had died though.
CLANNAD is just filled with inspiration. Though there are other inspiring stories from Key, such as Little Busters! with Friendship, Rewrite with Humanity, Angel Beats! with Life, Charlotte with Reality, Air filled with Hope, and Kanon filled with Promises. CLANNAD is the one that takes all of that into consideration and expresses it beautifully. I think CLANNAD is by far, one of Key’s strongest works, and most inspiring work overall. The music is gorgeous, the character designs are beautiful (I mean, not as beautiful compared to Rewrite), the CG’s are great (for something from 2004), the story is well-thought out, and the anime was great for Anime-Only Watchers.
I feel like nowadays in Key stories, the stories are driven by plot instead, which is great, but doesn’t have the same emotions CLANNAD had. CLANNAD was driven by development instead, which was a great way of showcasing its beauty.