Quality ranking of oil paint brands
There is no standard quality ranking for artist oil paints, however, generally, they are sold as Professional, Artist or Student grade with different brands having their own names. Price also is usually an indicator.

I think it is more useful to think of paint in terms of purity rather than quality.

The top end manufacturers are pigment makers firstly, the rarer the material and the more complex the manufacturing process the more expensive the pigment and thus the paint. (See Lapis lazuli, rose madder, mummy brown for examples). The most expensive paint contains one expensive pigment with oil added. A cheaper paint in the same range will be exactly the same quality only with a pigment that is easier to produce.

Budget paints will have two or more cheap pigments (and possibly filler) mixed together to resemble a single pigment.  

The oil is basically a medium to get the pigment from the tube to the canvas, cure, harden and then leave the pigments in as pure a condition as possible. Different oils are used, the most common is heat-treated linseed, I have also used walnut oil and safflower oil. The different oils have different effects on drying time, consistency, and finish, but this is a good thing as it provides flexibility and creativity which in my opinion is the essence of oil painting. 

I have read that safflower oil separates in the tube and that linseed oil goes hard over time. I would say that that is just a common problem that relates to ageing or poor storage.

You can find a list of popular brands looking online, if you are a student you could start with something like Monte Mart, Maries, or if you had a bit more money to spend you could look at the Winsor and Newton Winton range.

If you are serious and want to keep going you will probably be able to answer your own question by that time and discover brands like: Blockx, Michael Harding, M.Graham, Sennelier... I use M.Graham and Art Spectrum.