I'm not sure about Q.U.B.E. The concept doesn't seem so innovative to me.
The most creative game I've ever played?
I honestly can't choose one. But there are several that definitely changed my vision of the video (and indie) game world.
Portal. Not for the concept in itself, it's not really new, but more for its exploitation throughout the game. I can't really explain it.
Eversion. Oh my god. Just... oh my god.
Braid. Although the time travel has already been seen... But I'm thinking more of the storyline, the end of the game, the graphics...
Also, Braid almost made me cry when I finished it. It's the first time a game managed to. Braid is incredibly sad.
Inside a Star-Filled Sky ("Inception"? Pch. I can enter inside an enemy inside a power-up inside an enemy inside myself inside a power-up inside a power-up inside myself inside myself inside an enemy inside myself inside an enemy inside a power-up. Talk about recursivity.)
IaSFS has an utterly good concept. You can change things by entering in them, including yourself and power-ups (to improve them) and enemies (to remove their biggest powers). It's sad that it's not well-developed because otherwise it would probably have been the best game I would have ever played. Yes, seriously. It blew my mind.
Every. Fucking. Game. From Edmund McMillen. (Super) Meat Boy, The Binding of Isaac, Gish... This man is so frackin awesome.
While Super Meat Boy is a simple platformer, it's filled with awesome ideas, references and things that made me stay unmoved in my chair for almost an hour, thinking about everything this game had taught me in 20 minutes of gameplay. Isaac isn't less excellent. It completely renews the concept of the Rogue games, and does it like a boss.
Sequence. It's a RPG rhythm game. 'Nuff said.
Beat Hazard. A shmup where every level is designed by the music you play. For example, if you play a dubstep (ewwww) music, at the beginning you will battle against some enemies, and when the bass drops you are likely to face an epic boss. It's very, very well done.
LIMBO. Because black and white, little boy, creepy world, OMFG IS THAT A FUCKING GIANT SPIDER OH GOD OH GOD.
Awesome game. Not sooo creative though, but very well-made.
So yeah. I played a lot of very creative games. All of them are inspiring me at some point, and that's why my games are so fucked up now.