I can echo that it may not be the best in the series, but it's still being damn fine game in it's own right, the one thing that I appreciate the most however is just how genuine it feels.
Interviews and it being the first Good Feel developed game always gave me the impression that Shake It was very much a passion project and it really shows in the game itself. In it's gorgeous hand drawn visuals, the way it perfectly captures Wario's character as a brash, greedy treasure hunter, rather then reduce him to nothing but a FatFartFanatic and the various throwbacks to older Wario plattformers like the return of Captain Syrup, the Land 2esque train level, the Greenhorn Ruins remix, the subwarine levels (which bear some similarity to Captain Wario from Master of Disguise) and a couple of returning Land 4 mechanics.
Above all else I love Shake It because it shows that there are people in the industry that truly love and appreciate both Wario the character and his series, which is something we don't see often enough and that alone already makes Shake It something truly special in my eyes.