For portability, I was primarily using Larousse on my idevices for a while, but lately, I've been using UltraLingua more and more because their dictionary also comes with verb conjugations which is really handy. I still keep both apps though, since I find that Larousse seems to have more "colloquial expressions" type entries than UltraLingua. These aren't web-based apps, so I've been able to use them while traveling abroad.
For web-based, I like bab.la:
FWIW, I also have the Oxford Hachette dictionary in my bookcase, but I use it very rarely. I can think of only one occasion where that was the only place where I could the find the word that I was looking for (it was a technical term). More often, I use the Merriam-Webster French-English dictionary as my desk reference for writing because it's more portable.