I have too many favorites!
Boss DD-20 (tap-tempo, saved presets, great sounds)
Boss RE-20 (great reverb)
Visual Sound H2O (great buffer, killer chorus as well - I have two these)
Ibanez DE-7 (really good inexpensive delay)
Ibanez AD-230 Analog Delay and Flanger (It is a HUGE hunk of metal that fits in around 4 rack spaces but sounds wonderful!!)
If I had to limit it to one it would be the Boss DD-20. Or the VS H2O. Or the Boss RE-20. Or...