Wario Land II, although I will admit to having a nostalgia bias (it's also the only one I've completed). The first game felt a bit janky, and I hated Wario's design. Wario Land 3 I haven't given much time but it's a lot like the second game as far as I can tell. Now that you've reminded me I might give it another go. Wario Land 4 on the Gameboy Advance was fantastic. If they made a new 2D Wario platformer I'd be all over that.
Wario Land II was neat in that it had a bunch of branching paths. You'd play through the game until you got one ending then you'd have the option of replaying previous levels and finding the different routes. It was also a bit weird in that Wario was invincible, if you got hit you'd just lose coins. But you needed coins to play the minigame at the end of each level and each one you completed would unlock a section/panel of a picture. Once that was completed something special would happen.
Also worth noting that Wario being invincible didn't necessarily make the game overtly easy. Enemies would often have ways of blocking your progress and you'd have to work out how to get around them. And bosses always have a way of knocking you out of the arena, forcing you to start over (which was functionally the same as having your character die).