The delay is probably the most used effects unit by harp players. With the FX loop of the amplifier, almost all delay units will be suitable and also before the amp, a delay is not so critical. Especially if you play on a normal guitar amp.
Joyo Analog Delay
Joyo currently offers many good (often copied) effects. I do not know what is technically behind this, but it is a great delay for harp. Well adjustable, discreet sound, lets pass a good level and reflects the harp sound. Even got a real switch, what more could you want? My recommendation!
MXR Carbon Copy
A nice pedal that also sounds great. It lets pass a very good clean level and can be extensive adjusted - incl. modulating effects (chorus-like). Because it offers so many options, the setting is not so easy, that's the deficit here. The composition is very complex - and, as well as the Lone Wolf ... expensive.
Lone Wolf Harp Delay
In the reviews often represented as somewhat dull, I found the sound very pleasant and it is set easily. It lets pass a very good level of clean sound and looks simple by technology (I like that) and construction. Meanwhile, there are distributors in Germany. If you are searching for DIY information, look here: "small time delay".
There is a second variation of the Lone Wolf delay that promises more treble. The effect is barely audible and inside you can see ... two 1u caps were replaced by .1u, that's it. You can make that yourself. Or just go down with the value.
Harley Benton Digital Delay
From the extensive Joyo-series marketed by Harley Benton. The blue is ugly, but it lets pass a good level, relatively neutral, is well adjustable and has a true bypass with the correct switch. An advantage, if the device is put down in the back of the amp.
You can also build this pedal on your own, there is a kit at UK-electronic (PT80), but costs much more! The sound is pretty close to the Joyo Analog Delay.
Behringer VD 400
Behringer has not the best reputation, but I have some devices of this company, which are even partially superior compared to the big brands. The pedal let pass a good level. What bothers me is the button, I like a real switch which retains its state when you unplug.
The sound is a bit musty/tinny, which has not to be negative - "vintage" precisely. The names of the controllers are a little misleading. Referring to "the original", the Boss DM-2 (Delay Machine), which is traded for huge sums.
EHX Memory Toy
I'm a fan of EHX devices, but this disappointed me. It's obviously cutting the sound and it seems stilted. I don't like it.
This pedal has amazed me on the scope ... it takes only a little level and brings very little out. This makes little sense before a tube amp, if you want to use the input tube - right?
Oddly enough: It works and sounds quite nice! But I would not buy it for me.
The newer version of the DD3 that Gussow carries around. But it has an "analog" mode, which is a crucial additional benefit for the very clean delay. Like all Boss devices, it lets only a limited level pass, which possibly has a negative effect. The sound itself is good and it also has a small Looper in there. Whoever uses a weaker micro, can become happy.
Artec Analog Delay
My three versions all had wild noise and I didn't like the sound. Operated with 12V, the level is fine. The picture shows the version with (unnecessary) tone control.
We know it by David Barrett and can refer the problems of low input on the net, whereat it might has the advantage that the device is not driven into distortion by a strong Bullet. I think, it's too big and heavy, the buttons are tiny and mechanically sensitive. One can hardly decipher what to adjust there. Level-wise it ranks at the level of the Boss devices. The sound itself is good.
Jürgen, the one with the beautiful "Poor Bluesman", describes a modification on his site to a higher impedance input.