I got the most enjoyment out of Circle of the Moon compared to HoD, AoS, and DoS. (I haven't played PoR or OoE). It was the first game I got for GBA and the reason I got GBA on day one.
Nathan Graves was a cool character, it had a great soundtrack, a N64-ish moody visual style, awesome bosses, and good old-school whip challenges mixed with the modern Metroidvania exploration.
AoS is a close second out of those mentioned above, as it had an intriguing storyline and a new play-style with the Tactical Souls. HoD I found interesting, but too "all over the place" in terms of design. DoS was pretty, but was too much of a retread.
To be honest, though, I have trouble (interest-wise) playing through any of the Metroidvanias more than once, and I find it largely unrewarding to play that kind of Castlevania game at all now. By AoS, I think we'd seen about the best that format can offer design-wise without drastic revisions--which haven't happened. I'll probably still get PoR and OoE, but not for a while. And I think I'd get PoR first because it has the whip, ensuring some sense of the old-school.