Usually, I use either dict.cc or LEO as online dictionary or DING as offline dictionary. Both online dictionaries have the advantage of providing more information while the offline dictionary is, well, available offline and faster too. Recently, I discovered that dict.cc can be downloaded for personal use as well. As they have had some severe copyright infringement issues, where a company downloaded their dictionaries and claimed them their own, you will get a personal and traceable version of the current online database. The format of the file is exactly the same as the database of ding. (The database of dict.cc was originally based on ding’s database.)
This way, you can combine the best of two worlds: A comprehensive and current database and fast offline lookup.