Wow! I love a lot of those anime listed int he pole. I mean, its so hard to choose. I'm also going to start watching Legend of the Galactic Heroes, which I'm also sure will no doubt become one of my favorite classic anime when I see it, since it has a premise that I just love.
That said, here are some obvious choices for me:
Yu Yu Hakusho- If you haven't figured it out by now, this is easily one of my favorite anime, period. Its not exactly some highly original concept, but what's fantastic about it, just like Magus said, is its characters. The way they interact with each other, and develop, is just handled so well, IMO. I prefer it to most shounen, especially the more popular ones like Naruto, Bleach, D. Gray-Man, Soul Eater, etc. (though, I'm not really a fan of shounen, to begin with). Of course, that's just my opinion, but to me, YYH never gets old, and I will never get tired of watching it.
Rurouni Kenshin- Once again, pretty much what Magus said, however, it did SERIOUSLY get dragged down in quality with the last season, full of filler. The worst part is that the TV series didn't even get to animating the final arc, and IMO, the best part of the series, from the manga, which makes me wish that they would have just ended the show early, after the Shishio arc, rather than carry it on longer than it needed to be aired, and ruin so much of its quality that it had developed. But, at least the characters in the show were always awesome.
Outlaw Star- Its a great show. Simple as that. Also, while I don't like it quite as much as Cowboy Bebop, I will say that its a very different show and a very fantastic one that goes along with Cowboy Bebop among my favorite anime (out of what I have seen). It definitely doesn't deserved to be overshadowed by CB, even though I do find CB to be the better series, by at least a little bit, if I had to pick one over the other. Even then, that doesn't make Outlaw Star any less awesome than it is in my book.
Lupin III (Red Jacket Series)- Now I must admit, I haven't even seen half of this 100+ episode series, but I absolutely LOVE what I have seen of it. Its great classic humor at its best, and this show definitely deserves more popularity in America, and other countries besides Japan.
I love quite a few of the other classic shows, whether others consider them classic or not.