Definitely the best game in the series, bar none.
It takes all the best parts of Wario Land 2, and refines them.
This time around, rather then treasures just being used to unlock a secret level, the treasures themselves are the goal.
Unlike the previous game, Wario will not start out with all of his powers, you will gradually unlock them by collecting certain treasures as you progress through the game, you will need to return to earlier levels with these new abilitys to obtain more treasures, ala the Metroid series. However, your powers aren't the only things that will aid you in your quest for legendary artifacts.
Certain treasures will alter other areas so you can obtain treasures that were previously inaccessible even with your abilities. Furthermore, some treasures can only be obtained at certain times of the day. Yep, that's right, this game has a day\night system too!
There is LOTS of treasure in this game!
As a matter of fact, even when you get to the final boss, on your first playthrough, you will likely only have about half of the total treasures that you can collect in the game.
The controls are responsive, and intuitive.
The art style is a more refined version of Wario Land 2.
The coloring is generally pleasing to the eye, and contributes to the atmosphere of your environment.
Speaking of environments, just as in the previous game, you will romp through a wide variety of different settings during the course of your adventure, ranging from a forest, to a beach, the ocean, a volcano, an abandoned village, a frozen lake, pyramids in the desert, a castle, and those are just off the top of my head...
The animations are smooth and fluid, Wario's walk cycle alone is super satisfying to watch.
The soundtrack is as fantastic as ever, with the ending credits being one of the most beautiful pieces of music to ever grace the Game Boy Color's soundchip.
This game's only flaw is that STUPID golfing minigame you have to play in order to unlock some gated off areas of certain environments.
If you have ever played Resident Evil Gaiden, then you already have a rough idea of how the "golfing" in this game works.
Instead of having to time your button presses so that you actually hit the enemy you're aiming at, you have to time your presses twice, once to determine how far back you'll swing your club, and once more to determine how hard you'll swing it.
If you don't get the ball in the hole before getting above the PAR score, you will be kicked out and forced to start the minigame over.
UUUGGH!
Aside from that minor niggle, this game stands right up there with Pokemon Gold, Silver, and Crystal, as one of the killer apps for the system.