I still use Opera browser (Opera mini to be exact) on my Symbian phone (Nokia Pureview 808 with 'Belle SP2', a megapixel beast with 41 megapixel and shoot photos and videos quicker with very easy-to-reach configurations than most of the modern smartphones I played with).
Be frank the design of Opera mini has all the basic features offered in a very handy way with human-factor considerations.
On Linux & Windows: Firefox, because it offers syncing and many extensions.
On iOS of iPad: switching between Opera Coast, and Safari, and still wanting to try Firefox, but I'm super used to the previous 2.
On my Windows Phone 8.1: I'm never used to any of the browsers on WP8.1, be it Opera mini, UCBrowser, Internet Explorer (and perhaps it's not in such a one-hand thumb-operable style like those older pocketable phones like my Nokia 808 or the current iPhone 7)