Yes, I easily think Galaxy is the best one. It expaned on the concepts of Mario 64 and made them bigger and even more complex, but in a way that was even more fun and 'balanced'. And, most importantly, made it so you needed to utilize all aspects of Mario (and Luigis) actions to complete the levels and the game. In other words, it was one of those games that had a near perfect combination of difficulty and utilization of all the gameplay presented. A feat even Mario 64 didn't accomplish. Which is understandable as Mario 64 was one of the first true 3D games.
On a personal note, Mario 64 is still good, but back in the day and still now, I find some of the levels to be annoying or even frustrating, due to the camera or layout of the level itself. There's none of this in Mario Galaxy. Sure, you might die a few times from falling off a cliff or a disappearing ledge in the 'Matter Splatter' galaxy. But after a few tries, you get the hang of it and its not only fun, it challenges you to proceed on to the next level. In Mario 64, you will die many times to the same thing, but for different reasons. Be it the camera, losing your breath from coins or simply timing a series of jumps wrong. Things that were, for the most part, fixed in Mario Galaxy.
Overall, the name of the game is just balance. And Mario Galaxy has it all. You still have tons of exploration, but still tons of action and platforming. At a good balance. Where Mario Sunshine had you at the mercy of a waterpack with NO offensive value and Mario 64 made you rely too much on fighting your way out of things with a experimental camera (which, looking back, was still revolutionary for its time, but still can't win a gold metal for best Mario game ever).
Six upcoming games you should look into:
