Portable graphics have come a long way from Gameboy. Its pretty amazing what they are capable of nowadays and this is an appreciation topic for these powerhouses that come in small form factor.
This is all about graphics, not visuals, so only discuss the most technologically impressive games of the generation. So no DS games, because PSP had objectively better games in terms of technology. No 3DS games because Vita. Talking of Switch games mention the best looking game in portable form and not on docked.
I'm doing this to keep bias out of the way. Obviously many would say a DS and 3DS game because it "appeals" more to them visually which mocks the point of the discussion. Its appreciating technology, not art styles.