Can't really say what's the best sounding delay pedal because that's different for everyone.
For some, the best delay is the most sonically pure one that does nothing to alter your tone, and all repeat notes sound as crisp and clean as the note you play.
Other people want a delay that gets dirty with the repeats, adds warmth, perhaps does modulation, sounds more vintage, etc...
If you like vintage sound and have a parallel effects loop in which to place it, I think the Line 6 DL-4 has gorgeous sounds. It colors your tone alot, though, so it's best placed in a parallel loop so you can run some clean signal alongside it.
My delay of choice is the BOSS DD-20, which can sound studio clean, or you can do some good modulation effects, and it has some neat modern delay effects.
You can read a review I wrote on these, along with the very clean sounding Rocktron Cyborg, here:
http://www.musicplayers.com/reviews/guitars/2006/1006_DelayPedals.php
Scott