OK, I will start with two games close to my heart and a third that closely follows.....
Mysterious Song
Pyramid Plunder
Insanity
Beside the fact that I worked on two of those three games, these are MUST BUYS because ONE: They are great games and TWO: They are great examples of the living Turbo/PCE.
Now, if I were to go on to a list of classic games:
Gate of Thunder
Ys Book I & II
Bonk's Revenge
Dungeon Explorer II
Neutopia (1 or 2)
Legendary Axe (I or II)
I think that covers some of the best games across a few genres and doesn't leave the US released games. If you go into Imports the lists gets much longer. This list is also not REALLY pricey (Dungeon Explorer II pushes it up quite a bit). Also, it is a nice list of CD and HuCard titles.