Only buy feta made in Greece, this is the only one that is REAL feta. Feta is Greek and it is acknowledged as that by the European Commission due to a variety of factors. If you want the BEST feta, choose Greek.
Make sure it only contains sheep’s milk (and maybe some goat’s milk), rennet and salt. It should not contain anything else nor should it contain cow’s milk. Feta made with cow’s milk can crumble easily, may develop a sour taste and does not leave a pleasant aftertaste. Moreover, cow's milk (in North America) contains many antibiotics and hormones, lacking the nutritional benefits of sheep's and goat's milk.
Taste it. If you have the luxury of being able to taste the cheese before you buy it, than do so. There are 3 different types of feta based on the texture: hard, medium-hardness and soft. There are also different aromas: some are saltier, some are spicy and some are mild. This all depends on what area and in what kind of container the feta is matured. Most Greek people have a favourite feta and they always buy that one. We love Tripolis and Dodonis. There are many like Makedonias, Epirou etc.
Feta should be white, not yellowish in colour. If it is a bit yellow, that means that the cheese has been exposed to air outside of the brine.
Feta should have a tangy flavour and a rich aroma. It should not taste bitter, sour, rancid, chalky, or tasteless like most North American made feta that is from cow's milk.
Feta should have a few tiny holes on the surface.