>>12001346 (OP)
IV, V, VI and VII (including VII part 2) are the essentials. All masterpieces and quite different from one another. Only problem is that the further back you go the more archaic the controls/UI/general vibe gets so I'd probably start with VII (which, interface-wise at least, is SUPER easy to get into) and maybe work your way backwards. Unless you're willing to dive into the deep end with Apple II era CRPG controls in which case start from IV but I probably wouldn't recommend it.
Ultima Underworld is also a masterpiece and worth playing (as is Underworld 2.) It's almost it's own thing though and can be played at any time without any knowledge of the main series. Would also recommend checking out the original Wizardry and Dungeon Master too as well if you want the full history of CRPGs