Valhalla FreqEcho is surprisingly useful; I've had it for less than a month and I'm getting a ton of milage out of it already. And, it's 100% free! I find it takes real-time tweaking super well... It's good at making warped, scintillating sounds out of mundane inputs. If I have any secret weapons, it's probably going to be one of them.
This is going to sound lame but I find Logic's Stereo Delay great for 90% of my echo needs. Separate crossfeed and feedback controls are great, along with programable inputs (i.e. you can make the right channel route to the left delay and vice-versa, and then each can crossfeed back to the other as the delays trail off).
U-He Satin has a delay mode, but I haven't gotten the hang of it yet.
I will have to pick up NastyDLY because I love a good chorus echo.