A crack in time is my favorite. The extra challenge rooms were over far too quickly. Flying through space and exploring all the little planets was the perfect distraction between story planets, next to the always awesome arena battles ofcourse.
I still have to finish all 4 one sometime, if ever. It's the first R&C I quit playing halfway through, and I didn't even want to buy QForce after the demo. They might be spinoffs but without any new sequel they're dragging the name down anyway. It's still fun to play the first ones over again in the HD collection, but they better bring the magic back on ps4.