They all have some pretty serious flaws that would keep me from recommending them to someone that hasn't played them yet unreservedly.
CoTM has an unimaginative graphics scheme, bad level design, and a marginal storyline.
HoD has vastly improved art design in every respect except it's too bright and cartoonish, has long, tedious stretches of unused level space that makes backtracking a horrible chore, and is too similar to SoTN for comfort (i.e. feels lazy)
AoS is probably the most well-rounded of the three, with better graphics and art, a better balance between sound and graphics quality, and an innovative soul system, but overall feels a little off, owing to the bizarre premise set in modernity.
I guess of the three, I really would have to say Aria, although I personally have revisited Harmony more than the other two, which perhaps attests to its lower difficulty and ease of completion.