I know you said "rackmount," so rack mount a PC with a VST host loaded on it, and buy Ronin:

http://

This is quite possibly the best delay I have ever used, and no, I do not work for them.

Classic units:
Lexicon PCM41 & PCM42
Roland SDE 3000, SDE 1000 and SDE 2500
Yamaha D1500
TC Electronics 2290, M5000
AMS 1580
Eventide H3000, H3500, GTR4000,DSP4000, DSP4500

Decent newer units:
TC Electronics D2
Any new Eventide model
Nothing else I can recall that is worthy...

tc d-two is nice for the price!

Symmetrix made a great DDL several years ago, and it was a single-space unit with lots of cool mod options. I think it was called the 606 (?). The units that AudioGaff listed are definitely great units, too. I have 2 of the Rolands and a Lexicon PrimeTime...there isn't anything on the market today that has the user interface like they used to(IMHO). Very easy to operate and vary on the fly, especially for live sound or for that Brian Eno/ Pink Floyd/Fripp experimenting...PEACE.