I have Opera, Firefox, Safari and Chrome all on my Mac. To be honest, just to test the stability and useful features of all the browsers, I use them all on a daily basis. (Actually, except for Chrome. It not only hogs Ram and lags with many tabs open, but Google is fishy about the privacy of that browser anyway.)
I personally love Opera. Since Opera has convenient page previews that pop out of the tabs, i's really easy to pick out the right page, especially when I have a lot of tabs open. Opera also allows "Stacking" or putting tabs into virtual "folders" of tabs, so I can sort the tabs I like by category or relevance to me. Opera also features "Turbo," which speeds up web rendering speeds on slow wifi. All said, Opera is my favorite browser.
I like to use Firefox for certain things as well, but the recent (and still unresolved) issue of typing lag keep me away from using Firefox too much. Another thing that bugs me is that Firefox doesn't support Magic Trackpad gestures nearly as well as any other browser available.
(Good to note: Firefox 19 added a great in-browser PDF viewer. It's really convenient if you need it.)
And Safari, while Safari seems to be derided often, it's really not as bad as it's made to sound. If you're using a trackpad, the zooming and swiping smoothness aren't matched anywhere. Bookmarks and syncing with iOS devices is really nice, and that's really the majority of what I use Safari for, using iCloud tabs from my iPod, when I want to look at sites that I was reading previously. Otherwise, the weird tab "limit" (while not a true limit, the tabs stop displaying fully in the browser, and have to be found from a pop up) is really annoying for me, as I always have a lot of tabs open anywhere else, so it's tough to use Safari for regular browsing. If you do settle on Safari, I recommend glims for Safari, as it makes it easy to switch out and use different search engines besides the "regular" defaults. (I'm really partial to DuckDuckGo)
So there's my two cents, I strongly recommend Opera. It's the most well rounded browser of any that I've used.