From the ¨hat-era¨: the Dragon Cap. From the transformations? Hard to say, but I think fire Wario. There's jsut something funny about seeing Wario running hysterically while his butt is on fire.
King Dragon Wario from VB Wario Land is formidable, however I do like Bull Wario just as much. The horned viking-style helmet befits Wario's looting and plundering (although thankfully he leaves out the raping).