As many have already said, it is easily one of the best in the series. Now I cannot claim to have played every Mario title, I have played the majority of major releases. Galaxy is one of the most entertaining and edge of your seat platformers I've played in a long time. I am currently only 47 stars in which I believe is almost half way, and I cannot wait to get back to it(I've already put in almost ten hours today, which is almost more Mario than I can handle). With Mario jumping from planet to planet on his outer space mission you get the feeling like you are constantly on the edge of falling into oblivion, but fear not, most of the planets have a strikingly high level of gravity and will grab you right as you are sure its over.
As nearly all Mario games are very similar in plot I feel no need to really go over that in any detail what-so-ever, Princess Peach, Bowser, blah, blah, blah. But hey at least Peach sends you 1ups from her prison cell. Now I don't know what kind of country club prison Bowser is running, but the fact that Princess Peach the most highly valued prisoner in Bowser's castle can send unchecked mail to her boyfriend via a bunch of mushroom people seems highly irregular, and makes me seriously doubt Bowser's sincerity to his cause. I think secretly Bowser enjoys the cat and mouse game he and Mario have played out over the years, and without it he would probably be lonely with his castle full of spinning fireball turnstiles, and haunting music.
Anyways Galaxy is a great game, if you own a Wii it is absolutely a must have, and you would be crazy to pass it up if given the opportunity. Also I should mention the Wii as a next-gen system really shines with Galaxy, and really shows what the Wii can do both with their unique control scheme, and graphical capability(it ain't no Bioshock, but it certainly does shine). As a last note, the co-op mode is fantastic and makes the game a heck of a lot more fun, especially with someone to collect star bits, and hold down enemies for you. Well its late here, off to bed with me, much holiday shopping lays ahead of me in the morning.