On my laptop, when using Windows 8.1: Internet Explorer 11
On my laptop now, using Windows 10 TP: Internet Explorer 11/Spartan engine
On desktop:
• IE11 when I just want the fastest possible browser. It just appears the fastest to me. I also love the sync of tabs/password/favorites with my Windows Phone.
• Opera v28, because the Speed Dial is just too awesome
• Firefox 36 currently in use, because it isn't the most horrible mainstream browser in existence anymore. They introduced rendering on another thread in Firefox 34, so it finally doesn't freeze when you use more than 2 threads. It's also pretty fast and I love the sync.