I think Wario Land 4 is really the one that really sells the idea that you're playing a game about Mario's doppelganger. Mario games are linear platforming challenges, WL4 is a weird puzzle platformer with open ended exploration. Mario power ups give him neat superpowers, Wario's mutate him into hideous alternate forms. Mario soundtracks are poppy and melodic, this game's soundtrack is full of eclectic weirdness. Mario fights goofy looking cartoon creatures and a giant turtle, Wario's enemies are uncanny valley nightmares.

In case it wasn't clear those are all points of praise. Wario Land 4 is just a dub all around. Weird. Wild. Wonderful. Wario.