As a pure handheld experience, I have to go with CotM. It was the first CastleVania, after SotN, to do the Metroidvania style. It managed to feel almost as epic, and really nailed the score. At that time, GBA was pretty impressive hardware. The stereo sound capabilities helped it immensely. Did I mention that soundtrack!?
The lack of a backlight sucked, but the dark GFX really assisted with immersion. Especially playing it now on a 55" HDTV, no more sitting under a lamp!
My 2nd vote was a no brainer for me. DXC for PSP. I could fill a book with the adoration I have for the original PSP 1001 model. I was very active in the PSP Hardware modding community. It was actually the first time I got the original RoB to work thru an emulator, since I first experienced it back in '93. I basically forced myself to figure it out(Japanese Menus didn't exactly help) over the course of several days, but the payoff was sublime.
Needless to say when I learned of the remake, I was beyond thrilled. The package consisting of 3 top tier games was pretty amazing. The small differences in RoB were detracting, but the combined package is just too much gaming goodness crammed onto one UMD(disc). The value to hour ratio has to be tops amongst handheld CastleVanias.