Samsung F3 Spinpoint 500 GB or 1 TB have an excellent reputation. If you install a 2 TB and actually intend to use all that capacity, you will need an external drive -- stay far away from WD externals, although the Caviar Black internals have a good reputation -- of equivalent capacity for a clone, or much more for use as a Time Machine backup. The Samsung's are much better value. Probably not going to be around that much longer, at least in those designs, since Seagate gobbled them up.
Always a good idea to check out the reviews at newegg.com.
Stay away from any "green" drives. You can install up to 3 TBs (I don't think 4 TB drives are yet available.) Disassembling an iMac is not a trivial task. I would visit OWC to find suitable drives and tutorials. OWC also sells drives with toolkits needed for the specific computer model.
Personally, I'd avoid WD drives. WD is not the best supporter of Macs. I would stick to Seagate, Hitachi, or Samsung for large capacity drives. Stay away from drives that have built-in sudden motion detection as these conflict with the SMS in the computer. If a drive has a SMS then be sure you can disable it via software from the manufacturer or by configuration jumpers on the drive.