There are ONLY two possible and viable answers for this question, period.
Skate to Skate 2. I have NEVER seen so many additions to a game in my life, EVER. The ONLY thing they didn't add was my beloved darkslide, but they added that in Skate 3. Massive improvements in camera placement/control, new moves, park design, etc.
Star Control to Star Control II. They took a decent space battle game and made a giant, sweeping, adventure-like driven story with even more ships/races and TONS to do in the sequel.
To the mattresses. 3DS code: 2621-3902-1737, PSN: Justice_98405, Xbox Live: Justice98405