boss dd- series are rubbish, don't like them at all, don't like their idea of delaying sound. waste of money
digitech digidelay... well, cheap BUT ok. I'd save money for something better, but for the beginning - it's ok.
line 6 is not so bad, bypass is decent. i was using it for about 1 year. Good for gigging, would not use in professional studio
I love electro harmonix pedals, have bunch of them, but personally - EH delays are really nice with guitars... and that's it!
there's ONE delay - boss re-20 space echo. I fully recommend this delay, so sweet! bypass is not so good, but who cares?...