No contest: Reverb.
I've come to regard it as one of the most versatile effects for harmonica.
I'm very happy with my DigiTech DigiVerb pedal - it sounds great and is
better than the Song Works Li'l Lanilei (POS that it is) without question,
and better than the more expensive Boss Digital Delay pedal which gives
harsh and nasty synthetic break up. The DigiTech sounds like an authentic
analogue effect without the down sides - although I'm willing to believe
there are better quality effects than this, for the price and the size its
great for my needs.
-- G.