VVVVVV has extremely simple and polished gameplay, what makes it interesting is how the same gameplay mechanic is constantly challenged and reinvented through very clever level design. It looks like metroidvania as far as exploration is concerned but is the complete opposite in general progress, as in the only thing that locks you out from an area is pure player skill, reinvesting what you learned to go further: "oh, you can do this" instead of "oh, I unlocked this and so can now access that".
It was an instant classic for me.