The 2012 -November 2016 non-retina MacBook Pro is the most flexible and ultimately had the potential to be the most powerful laptop Apple sells, because it used nonproprietary, upgradable RAM and the CD drive could be easily replaced with many types of solid state hard drives with minimal effort. The battery was also screwed down, rather than glued down, which made it easy to replace without hurting yourself or damaging the computer's case.
The November 2016 refresh killed off that computer for good. The new MacBook Pros are a performance upgrade for a product line that had gone four years without a significant redesign, but the move to smaller, thinner, sleeker has sacrificed customizability, repairability, and upgradeability.
The new MacBook Pro, like its earlier Retina designs, has a glued down battery and has RAM that is soldered into the computer's logic board. Unless you're an expert microsolderer, the specs of the computer you buy are the specs you'll have until the end of its life.