IE: Opens faster than any other on a Windows system, The "Favorites" management system is really cool and simple. Since it stores them into files. It doesn't manages very well a large number of TABS, for example +10.
Safari: Has a nice "Search" function which highlights the text you are looking for with an opaque background and yellow keywords. I like the User Interface, but as always It's the slower (along with Chrome Google). It takes ages to open. It has a nice rendering engine (like Clear-Type).
FireFox: Of all three, I think this one consumes a lot of RAM. But performs flawlessly even with +20 TABS opened. It has a very good CPU management. It has the ability to support add-ons, some are cool, some not so much. I didn't like the SQLite integrated support for Bookmars and other things. I like the download dialog too, It's one of my favorites.
Opera: Simple, fast and clean user interface. If I can remember this consumes the lesser RAM. Has an in-built e-mail client and .torrent download support. Since new version it has a new rendering engine. And I like the home page shorcuts with previews (like in safari).
So, to resume, it depends on what kind of surfing you do. If you surf a lot of pages and do a lot of things at the same time on your computer I would recommend FireFox.
If you do a few things, but like to see the Internet windows loads fast. You like simple bookmarks (which you can manage with your Windows Explorer, just like files). Then IE is for you.
If you have a fast machine and plenty of RAM. (A nice processor, like 2.2 + and above, dual core, and 2gb of ram). And have a nice graphics card and do like the Cover Flow of safari, then it's for you.
If you would like something different, simple, fast go for Opera.