I used to think that Galaxy was inferior, but no longer do I.
You guys compare its difficulty to you now to the difficulty of Super Mario 64 and Sunshine then, when you were younger. Games are more difficult when you're younger. I say Super Mario Galaxy is as good as SM64 and Sunshine, as each had their own, unique charm.
Super Mario 64 was very fantasy like, based in Peach's castle with quirky tunes and very kid-friendly content. It was the "all-around 3D Mario game," because it focused on a very diverse premise.
Super Mario Sunshine took a leap from SM64 with a tropical setting, and a very different type of platforming, one that was based around the F.L.U.D.D., rather than Mario's own prowess. It had its own charm, that was very distinguishable from SM64, enough to be reckoned as a totally different game, and equally as good.
Super Mario Galaxy, the latest in the series does exactly what Sunshine did - but in a totally different way (oxymoron). It took the idea of SM64, spun it on its axis and threw it in space, along with of course, an incredibly enhanced physics engine, and totally new ways to play as Mario. It wasn't as farfetched as Sunshine was, but neither was it as grounded. Super Mario Galaxy is by far, the most mature in the series, as it has the most involved and mortality-influenced concept.
All in all, I find these games to be some of the best ever made, and I praise them all equally for their own merits, not the battle between their aspects. Why war these gems with one another when their all incredibly fun to play?