Western Digital have three main types of HDD: green (energy efficient, perfect storage drives), blue (good low-cost all-rounders) and black (high performance, higher power usage, perfect boot drives if you can't afford an SSD). If you're only getting one HDD for your PC, you'll want a blue or black depending on budget and performance requirements.
SATA 1.5 Gbps, 3 Gbps and 6 Gbps are all forwards and backwards compatible so you don't have to worry about getting the right type. HDDs are only barely pushing the 1.5 Gbps limit so whether you get a 3 Gbps or 6 Gbps HDD really doesn't make any difference. The SATA controller it'll be connected to is more important (Intel and AMD ports generally being faster than VIA or Marvell ones).