Wario Master of Diguise is awesome, creative, unique and somewhat fun, but it's the worst we have so far.
Wario World was perfect until it the end of the game, but that's the problem, it's too short, and playing the game again is kinda useless.
Wario Land Super Mario Land 3 is heavily based on the Super Mario formula, plays nice, has funny graphics and a good soundtrack, but it feels slow and heavy... some powerups are just a pain and feel more like a hazard than a reward.
WarioWare Smooth Moves was one of the most fun experiences I had on the Wii! However it's not the kind of game you really go back to unless you like to move all the time, some controls are irresponsive and compared to others, this title drags a bit. It just get boring.
WarioWare Snapped is basically a short showcase demo. It was short fun, nice concept but that's it. You play it once and you're done. Plus the camera capture is bad... very bad.
Wario Land 4, although it's my favourite, it's short, like Wario World. But at least you can play it on the go at least 3 times on each difficulty.
Now I could go on, the Wario series isn't perfect, but what makes a game good or bad is what people like or dislike in the experience.