Hmmm...I'd say Sailor Moon because it was my first anime. Fushigi Yuugi's OP theme also creates a huge wave of nostalgia as that blue and red lightning clashes The first Card Captor Sakura theme has the same effect ^_^ All of my most memorable shows were cel-animated, so I'm very excited to be getting into collecting now after all those years.
Hmmm... although shows like Ahiru no Quack and Ox Tales come to mind, the show that probably makes me feel the most nostalgic is Pokémon. It was really big in my country 12 years ago, everyone LOVED it. I've seen the first few seasons as a kid.
I have kind of an interesting reaction, as my nostalgia tends to be triggered by music rather than by the actual images. I can get very nostalgic when I listen to:
The Saban Animation theme to "Grimms' Classic Fairy Tales"
Hey, come along and join the fun.
It's a time for fairy tales.
Magical kingdoms in the sun.
Come along, come along...
That was the first really engaging anime my daughter (she was only six) and I watched together. We had no idea it was anime.
Also Belldandy's and Keiichi's image songs from the AMG "Singles" and OST 2 collections. They are based on themes from the OVA and, truth to tell, I always liked the BG music much better than the plot or the animations (also liked the manga better than either anime). I think "Megami wa itau" is a stunning song and love the simple twin guitar accompaniment to "Boku to ikiru koto ga kimi no shiawase." (And I can be prevailed to listen to the electronic version with Belldandy taking the song as well.)
Like KuroiTsubasa4, yes, the first opening theme for CCS, plus Tomoyo's wonderful song "Yoru no uta" or "Nocturne" (words and music by Nanase Ohkawa).
And I simply adore the opening theme of Haibane Renmei -- not "Free Bird," but the piano piece you hear at the very start of the series, "Refrain of Memory." I don't know if it's nostalgia, but it always makes a thrill go down my back.
This is so making me go dig out a bunch of CDs to put back in my car! Interesting thread. Thank you, Keropi.
Fushigi Yuugi - It's the anime that hooked me on anime for life. Every time I hear that opening theme I feel nostalgic about my love of anime. Sailor Moon also makes me feel nostalgic but for a whole different reason.
Most nostalgic? I would say the Super Robot genre, Daimos and Voltes V most of all... However Voltron comes a close third and although Macross is not a super robot series and more mecha... Yeah that too.