I think this depends massively upon the song in question. That being said however, I think reverb. IMHO, reverb is more "general-purpose," if you will, while delay is rather song-specific.
Something else to consider: you said any kind of delay. Reverb can be considered a delay with rather peculiar parameters. And if you set the diffusion on a reverb low enough, you'd have a delay. So they're the same thing
