The Wario Land series is certainly interesting, as it evolved from the Mario Land series but took the direction and design in a much more interesting direction.
I rather enjoy the original Wario Land- the main character's sprite is massive, but the game design compensates for this with predictable enemy patterns and organic design. The coolest thing about this title is its two examples of evolving world maps and altered level design. However, its pretty clunky and the actual platforming doesn't get all that difficult. The rocket and dragon powerups are totally awesome, though.
Of course, Wario Land II is a pretty interesting sequel that sort of continues the thematic elements of the original title, but has a vastly different design. It's post-game "twist" of alternate endings and level routes is interesting, but sometimes the methods for executing these alternate level endings aren't very transparent. In this era of the internet, it's not much of a problem, and that allows players to focus on the strength of the game- throwing obstacles at the players that impede process without using death as a punishment. While this might be frustrating to some, it is pretty clever to see how this is executed, and its a rare example of alternative design in platformers. The expanded moveset for Wario is pretty fun, too.
Now, I know many people argue which Wario Land game is the best- lots of people praise Virtual Boy, which I admittedly have not played, but it often comes back to Wario Land 3 and 4. In my opinion, in terms of charm and thematic design, Wario Land 3 greatly outshines its successor. Its evolving traversal elements allow revisiting levels to feel fresh, and the day/night cycle as well as the treasure chest system make every single level's evolution something enjoyable. I also greatly prefer the boss design in this game.
But if you play the Wario Land series for its platforming, then Wario Land 4 is likely your preferred choice. I'm not sure I like the return of health, but Wario's animations are clean and full of character in this game, and the level design is very thoughtful, although not all that visually appealing. I think the bosses in this game are absolutely aggravating, especially in how they can be surmounted through spending currency. Although, I think the idea of the key switches is neat and the concept is thoroughly-explored, the game didn't necessarily have to be structured around it. Still, Wario Land 4 is a great platformer and an awesome game, but it doesn't have my favorite elements from the series on display.
I've played, but not completed Shake It! and Wario World, and can say a bit more about the latter. It's weird, but there's something rather enjoyable about beating up enemies with the silly combat system. However, the level design is very strange, and the physics leave a lot to be desired. Shake It! is gorgeous and features some very fun level design- I wouldn't expect less from Good Feel- but the shoehorned motion control levels are insufferable.