The 17 cMBP for me, for the following reasons:
-Screen is large enough: Can have Safari window and Word doc side by side. Handy while attending school; could give small presentations comfortably when a projector was not available. Movies and pretty much anything else looks amazing on the large screen, and can share it with others. Music software plugins, home recording software, terminal windows, etc have more than enough real estate. Not to mention, it's easy to see.
-iMac replacement. I have a 27", but the 17 would be all the computer I'd need.
-Internal storage flexibility: I replaced the optical with another HDD in RAID 0, providing decent speed, future upgradeability, and massive storage to accommodate a family of 6's apps, photos, videos, and music. SSDs in RAID 0 would pretty much make it the fastest portable around.
-Connectivity flexibility: Got everything except USB3, but the Expresscard slot allows me to expand it infinitely (currently, I have an ESATA card in there for some legacy externals I have)
-Weight/Dimensions: 6.6lbs is not too heavy for me (and still the lightest 17 around). ~1-inch thin means it's nice and sexy and manageable. It has traveled with me around the world, and I've used it comfortably even in commercial flight tray tables. Great for military deployments where it's your only entertainment device.
All that said, it's basically irreplaceable. A Retina just doesn't cut it, yet. Maybe when storage gets large and cheap enough.