I'm a Firefox fanboy.
I dislike IE for its incompatibilites (6, 7, 8, 9-in-quirksmode; only 9 in regular mode is ok), its totally horrible settings/options/properties, its "yes, spam me with all cookies you have" and its security related issues which don't get fixed in ages.
I dislike Chrome for the "Google wants your data and tracks you" issue. From a technical aspect I heard many good things about Chrome and Google puts a lot of money into its fast development.
I dislike Safari as I don't intend to use an Apple product, ever, because I hate the fact that Apple believes it knows what's the best for me. That and Quicktime on Windows was a Frankenstein piece of user experience for me.
I haven't tried Opera.
I haven't tried Chromium, the non-Google, open source variant of Chrome.
Reading over this again I think I'm not only a Firefox fanboy but even a grumpy old man...