What drive should i buy?

I've been using an HGST 4 TB 7200 RPM drive for about 1.5 years, and I am quite happy with it, with about 176 MB/sec sequential reads/writes on CrystalDiskMark....(and they have fallen in price from $160 to $120 since I bought it, which is good for new buyers!)

I'd buy several more if I had an excuse to assemble a NAS...