Few points based on my observances of having owned all the devices listed here. 1)
Galaxy S2 is the only sane choice outside the Sensation today. 1 GB of RAM makes multi-tasking utterly trivial. The large screen is a delight to use. The phone is light, well engineered and thanks to it's rubberized back an utter joy to use. It can hold a massive 64 gb of space (onboard + cheap sd) There is a vibrant community of developers around it and all the major roms, are available for it. Also since it comes with an unlocked bootloader, switching roms or even flashing standard
samsung updates is as challenging as learning to breathe.2) I will agree with Sushubh on the Sensation. - If you are not getting a S2, you are getting this. It's basically the S2 with 256 mb ram less and no onboard storage. Really fast and it's bootloader just got unlocked last week. Expect the ROMS to flow. Otherwise outside the
iPhone 4 build quality, NOTHING comes close to the reassuring design of this affair3) Xperia ARC- Highly underrated phone. As light as the S2, 4.2 inch screen that's just mindblowing and currently the best camera among all mobile phones, outside the
Nokia 12 Mp affair. Slightly hampered by low amount of RAM but nothing bad for casual to moderate use. If this was a dual-core, everyone would be quaking in their boots.4) Blackberry... - Do not touch. The 9900 is based on OS 7 which is not compatible backwards with OS 6/OS 5 apps. Dead. No real reason to get this apart from BBM. Surprised no one mentioned Nexus S.