Neon Genesis Evangelion, partially because it was the first anime I watched, and, by being the first anime I watched, I was a bit too young to understand wtf was going on in the end. So, really, it was just this mind-blowing experience where Young Me had no idea what she just watched but she was very, very sure it was something extremely important.
Since rewatching I understand it a little more and love it a little less, but it's still something that's not going to go away ever.
I have a lot of memorable anime(So i can't choose only one^^"):
-Higurashi no naku koro ni/kai
-Puella magi madoka magica
-Neon genesis evangelion(especially the end of evangelion)
2) Final Fantasy VII: Advent Children/ Advent Children: Complete
3) Spice and Wolf/ Spice and Wolf OVA/ Spice and Wolf II
5) Sengoku Basara/Sengoku Basara 2/ SB Last Party [/Favourites]
Hajime No Ippo
Code Geass R2 School Days (lol)
Clannad After Story
Great Teacher Onizuka/Shonan Junai Gumi
To be perfectly honest i'd have to say Pokemon, followed by DBZ... Not because they're my favourite animes (which they're not) or because i've voted them 10/10 (which i havent), but because i grew up watching those, so they had a pretty big impact on me growing up, thus they're my most 'memorable' animes.