They've each got their own special qualities...I think I like classic better just because I grew up with it, started playing in grade school in like 2001 or whatever. Can't get into RS2 because it's geared towards a completely different audience, I find it's more for kids than anything. They took a sort of WoW approach where the original had things happen and you just had to figure it out and deal with it. I liked that, it was a challenge. Then as the game evolved it became easier and easier, until now they just sort of spoon-feed you everything. Same happened to Runescape.
I also don't like a few of the animations. They aren't fluid and are so obviously cartoony. I guess it's a bit tough to get that right when your character can't do anything after their attack until the opponent attacks.
Anyway. Classic has that vintage feel to it that I really like, plus just the overall simplicity of things. Take combat again, they overthought it a bit too much in RS2 but in classic it was very easy to understand, to remember the stats of weapons and armor, and generally just more appealing.
I was very disappointed when Jagex decided to butcher classic completely. I know there's private servers but the only thing people are interested in on those is pking which was definitely not the best or main feature of the game.