A while back I installed Chrome, Opera, and Firefox. I compared all 3 plus IE, and overall I liked the feel of Opera the best. It is fully W3C compliant (like this site is, look at the icon at the bottom of every page) If a site doesn't work properly in Opera, then it's not compliant.
I didn't like the minimal feel of Chrome, and Firefox was just too similar to IE in terms of interface.
Some of the benefits of Opera: Opera synchronize maintains your bookmarks across all devices (my desktop, laptop, and blackberry), Opera was the first to have speed dial (9, or more thumbnails on starting a new tab/page, also Opera was the first browser to have tabbed browsing, and Opera is the oldest and most mature browser of the bunch; started back in 1994. I also like the personal bar of Opera, and the thumbnails of the web pages on the tabs.
More and more browsers are copying Opera's features but Opera pretty much had everything first. If i were to chose a 2nd place I'd say Firefox would be my runner up, I just really dislike chrome and IE. I like the support that Firefox has, as well as the plug-ins. Opera has the most support out of the box (IRC, e-mail, RSS client, and even recently, an HTTP SERVER!) It's arguable that you should only install what you need (like with Firefox) but its more user friendly the way Opera has it and you don't have to worry about finding plug-ins and updating them all the time. I've got enough system resources to spare.