I bought the Castlevania Double Pack which was released late in the GBA's life. It includes Castlevania: Harmony of Dissonance & Aria of Sorrow. Both games are excellent but I enjoyed Aria of Sorrow a bit more as it was technically superior and had a more compelling story. As good as they are, they don't really stand up to Castlevania: Symphony of the Night on the PS1 or Castlevania X Rondo of Blood on PC-Engine/Virtual Console - which is my absolute favorite in the series.
Nevertheless, don't let that put you off. The double pack is worth picking up if you can find it at a reasonable price.