The highest performance 1TB mechanical HDD is the Western Digital WD1000DHTZ 1TB VelociRaptor. Price is $229 USD from Newegg.
SATA 3 (e.g. 6.0 Gbps SATA) won't matter for any hard drive, not a single one on the market can even provide burst speeds above 3.0 Gbps levels, not even the VelociRaptor. You need to get an SSD if you want the interface speed to really matter.
If you want something a little less exotic, get a WD Blue 1TB single platter drive. It's faster than the 1TB Black edition, and costs less.
For what it's worth, unless you're using a drive with FDE built-in, it won't matter if you go with a SSD or a HDD. Either one will support OS level software encryption, and the SSD has no disadvantages with something like Bitlocker or Linux-based FS encryption.