I believe The Beatles: Rock Band came the closest to being perfect. Eveything about that game was just beautifully done and the only things missing was Pro Drums, an option for Keys, and a few more Beatle songs (Hey Jude, Strawberry Fields Forever, Yesterday etc. etc.)
Undertale, for me. I have 0 problems with the game. Art style is great. Controls are great. Story is likely the most compelling I personally have seen. Not to mention the very appropriate humour.