Your max transfer speed will cap off at around 300MB/s with SATA II. The average HDD read/write speed is around 80 - 160MB/s, so if you're committed to a HDD instead of a SSD, you wont really be losing any noticeable performance at all.
What's your max budget? For that much storage space, you're looking at around $175 minimum for 6TB, and $190 minimum for 8TB.
Your best resource for narrowing down a drive is PCPartPicker. I've narrowed down a list
here. This includes 5-star rated and unrated drives (only reviews from PCPP are shown, each listing has links to retailers that host their own reviews. Be sure to check there if a drive you're looking at doesn't have any PCPP reviews). I've also set it to 7200RPM, which is the highest speed drives of those size can get, and a 256 MB cache.
The best way to narrow down your choices further is to set a maximum price, and a minimum/maximum storage capacity.