Probably Black Butler. It was the first none SoL, subbed anime I saw way back when so I remember being completely amazed by it.
Of course the manga is 10x better, but hey, the anime will always have my heart even that train wreck of a second season was entertaining as hell so no major complaints
I think Clannad After Story, there's other anime I consider a 10 enjoyment wise but I didn't give them a 10 because that is literally a perfect anime, but ones I consider are: After Story, Kill LA Kill, and Higurashi.
Toradora, my all-time favorite anime. Watched it when I was somewhat new to anime and was totally into Romance genre. It had great OSTs, emotional scenes, the ending was perfect, overall was a perfect anime for me.
My first 10/10 was One Punch Man, that was the year I created my MAL account I guess. It still is one of the anime I go back to and just enjoy. There were other animes I watched before that, but nothing like that except the long running ones.
I don't remember but I think it was Nagi no Asukara, I really love and enjoy this anime. It's rare I give an anime 10/10 honestly... I don't know why I give 10/10 to all three seasons of Kuroko no Basket xD