The word euphonic is itself euphonic, and here’s why, I believe: It has a stressed second syllable: you-FAH-nick (in the US, that is—see the blue image above).
I can’t explain why (maybe you can, in the comments), but I feel that words with three or more syllables and a stressed second syllable are often the best-sounding words. For example*:
emphatic
vehicular
bombastic
calamity (my favorite word)
exculpatory
recidivism
velocipede
rapscallion
infantilize
barbaric
insidious
pernicious
voluptuous
etc., etc.
Sure, there are plenty of great monosyllabic words—shank, shiv, minx, jinx, and so on—but when I look at my ever-growing list of favorite words, I’m struck by this second-syllable phenomenon.
Phenomenon: There’s another one. Phenomenal, right?