We'll have to wait until the 11th for Prime, but this is my take.
iPhone4S - Best screen (for now) in quality not size, best camera, probably close the fastest CPU, by far the fastest GPU, improved 3G on AT&T only (lawl), Siri is just software and I bet it gets jailbroken, iCloud is nothing special, and great battery life.
Galaxy S II - 2nd best screen (for now) that is a good size, good camera, perhaps the fastest CPU (will have to wait and see vs the A5), the GPU is old (some phones have had the SGX 540 for 1.5 yrs now) but adequate, LTE/WiMax, will undoubtedly be getting ICS, Nav, NFC (some models).
Nexus Prime - Looks like it will have the best screen (720p and over 300 ppi) but at 4.65" that sucker is gigantic, and I'm really nervous it will be just too big, but look to a curved design to compensate. Also, ICS does away with the 4 buttons, so perhaps the phone will be more like a 4.2-4.3" phone of today as the screen might extend the full length of the face. If it does have the rumored OMAP 4460 then I don't see how the dual 1.5 GHz won't be the fastest CPU out there. However, lets pray that they get the dual core 540/543 SGX Power VR GPU which will match it in speed with the iphone4S. A single core SGX 540 would be a tragedy on a phone with this resolution screen. The phone ships with ICS, and expect all the goodies (NFC, BT4, aGPS, WiFi, LTE). Camera will probably be 8-10mp. I'm hoping/praying that we will see a 2000+ mah battery for some decent LTE numbers.
Also, its rumored that this device may be VZ only (which is why VZ didn't get the Galaxy II), which while friggin awesome for me, sucks for everyone else.