I'd have to say that Smithy is the Mushroom Kingdom's greatest threat. Bowser's no slouch either, the guy wants to rule TMK with an iron fist and enslave the mushroom people, but Smithy was hell-bent on leveling the place. I'd say Smithy WAS (because he seems to have slipped into a blackhole since Mario RPG) the biggest Mario baddie, but good ole' King Koopa is just barely a step below Smithy as far as "evilness" is concerned. Here's how I'd rank the Mario villians in order of most dangerous:
1. Smithy
1. King Bowser Koopa (Bowser and Smithy are tied)
3. Tatanga
3. Wario (Tatanga and Wario are tied, too)
5. Wart
6. Donkey Kong (Because he's old and gray now)
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Have a nice end of the world.