DICT.CC – my favorite, although I use both this one and Leo (below) when I'm translating. I used Leo for years and also love it!
LEO – Seemingly everyone's favorite.
BEOLINGUS – A student with a technical background prefers this one. Maybe you do, too?
COLLINS – German and four other languages.
Abbreviations – very important in German! – From Abk. (Abkurzung) to z.B. (zum Beispiel).