Honestly, I can never anwser questions like this without stuttering.
But if I had to say which one's my favorite, it's a tossup between Wario Land II and Wario Land 4.
Wario Land II shares the spot because the levels are so creative, yet so fun to explore. The fact that you can't die also makes it stand out amongst other platformers, which I highly enjoyed, because it let me explore the level however I wish without worrying that I can die. The coins also play a much bigger part in Wario Land II, which is really great, considering you're playing as a greedy anti-hero.
Wario Land 4 shares the spot aswell for just being a wonderful platformer. I personally started being a fan of the Wario series with this game, so it's a no-brainer it'd share this spot, but I have many more reasons why I like it so much. The attention to detail, the hurry-up sequences, and so much more make it a absolutely wonderful game.