When you're deciding which hard disk to buy, it's important to think about how it will mainly be used.
First the 3.5-inch disks. If you will only have one hard drive in your PC, so it will also be your start-up disk, then it's important to buy a fast one. The Hitachi 7K4000 is our favourite 4 TB disk, and if you're looking for a 2 TB or 3 TB model you should check out the Seagate 7200.14 disks since they're also quite affordably priced.
If you have an SSD for your OS and software, and only need the 3.5-inch disk to serve as storage, then it's not necessary to spend money on the fastest hard disk out there. The Western Digital Caviar Green 3 TB and Seagate Desktop HDD.15 4 TB are good options then, they're affordable and relatively quiet and energy efficient.
For a NAS we would recommend getting one of the Western Digital Red disks, despite their higher price. The TLER feature definitely has added value, and the disks are both quiet and energy efficient, which are qualities you want in a NAS. And they're not that much more expensive than standard hard drives.
If you want an affordable storage solution for a budget PC, a good option is the Toshiba DT01ACA050 500 GB. It costs the same as many 250 GB disks, so you get twice the storage and very reasonable performance. For around £50 you can buy a 64 GB SSD that can serve as start-up disk.
If you need a hard drive for a laptop you're limited to 2.5-inch models. If you don't have the option of adding an (mSATA) SSD, you should definitely go for the Seagate Laptop Thin SSHD 500GB. It's significantly faster than hard disks without SSD caching, and it's not that much more expensive than standard 2.5-inch disks. The Laptop Thin SSHD is also very silent and energy-efficient. For performance it's a better investment than a 7200 rpm laptop disk.
If it's possible to combine an SSD with a conventional hard disk, a good choice are the Western Digital Scorpio Blue 750GB or 1 TB. It's affordably priced and offers good performance.
Many of the hard disks for this review were made avaiable by Informatique.nl.