"Best" tends to vary from person to person, and from situation to situation.
For example, I don't think that Opera, overall, is quite as good as some of the competition. But it is extremely good as re-flowing the text after I use the two-finger zoom on a webpage. (I don't want to have to scroll from side to side to read each line.) Since there's one website I have to spend time reading about 3x per day, and it requires me to do the zoom thing, and Opera's the best browser I've found for re-flowing text (at least on my phone and on this website), Opera's the best browser. For me. For this particular need.
Other people may need a browser that handles flash content really well; or that has a ton of available add ons; or that makes it as easy as possible to sync bookmarks, browsing history, tabs, and passwords with their desktop browser; or whatever. So, for them, some other browser might be best.
And for those people who don't know what it is they need in a browser, well, they might just be better off using their phone's stock browser until it proves inadequate in some way (assuming it does). Or perhaps reading browser comparison reviews and downloading/installing a few browsers which seem to have potential, and then deciding which browser best meets their needs.
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Michael