Two favorites here, but they are different animals, IMO. Both great for different goals.
Primo is more anabolic, Masteron is more androgenic. As far as things you'll notice visually, more fullness from Primo and with too many carbs in the mix, more puffiness. With Masteron, more hardness and strength, but less LBM unless the dose is pretty high.
Of course, most often you stack these with other AAS, so the LBM can come from another place (another AAS).
I like Masteron/NPP/Low test a lot. Hard, full, good LBM gains, low bloat.