Symphony of the Night and games after that aren't included, because that's where they started adding RPG elements to all the games along with non-linear progression. And this is where, I think, the era of classic Castlevania ends. I am aware that a few less known titles aren't included either.
For me, it's Castlevania: Bloodlines, the uncensored version naturally. In my opinion, the control was the best in that one, out of all the classics. The challenge is just right with three difficulty levels, and the music is the very best you can get with the Genesis sound chip. Traveling in many locations around Europe adds to the atmosphere, too.