So for me the meaning of "Must See" refers to things you have to witness to fully understand that thing (genre, type, ect.)
So my list had to be picked from those titals that influanced my love for Anime. That said a few of these may be a bit old.
1: Akira
2: Vampire Hunter D (actually both the original and BloodLust are really good
)
3: Dragon Ball Z (I mean seriously, who hasnt seen this series??)
4:Bubble Gum Crisis
5: Ghost in the Shell (the series is good, but for me the movie really influanced my love for anime)
6: Record of Lodos War
7: Spirited Away ( this movie just feels so nostolgic to me, its like something i would have seen when i was a kid, oh wait it was lol)
8: Escaflowne (I know its sorta shojo but that doesnt detract from the fact that the story is very good)
9: Higurashi no naku koro ni/kai (seasion one and two, good story, characters are easy to like, also i know this isnt old, but its good enough to go in my list)
10: Neon Gensis Evangellion
Anyway i know that may seem a little diverse, but to me those titals have to be seen to fully understand the things that make anime so great. Oh if i was able to go to 11 i would also add Cowboy bebop, but i saw that a good deal after i was already watching anime so it didnt influance me into liking anime more.