Yeah, Bowser is probably his most hardcore in Paper Mario and Galaxy. If you want to count the cartoons, he did wreak some havoc in the real world a few times which was kinda threatening.
I think it was mentioned earlier, he was pretty pathetic in Mario Party. An evil monster king, and he gets reduced to some misanthropic, antisocial jerk who has nothing better than to crash parties. Though to be fair, the parties often involve insane mythological stories revolving around stars and wishes and with that much on the table, he was mildly menacing.
But really, Mario Tennis on N64 takes the cake, considering how he drops in and scares everyone, but then smiles and basically reveals "Oh hey guys! I'm totally not here to attack you or do anything wrong. I'm just here for a good old fashioned tennis match. Sure we're archenemies, but let's put that all behind us and be friends for the rest of this tournament." I mean come on, at least in other sports titles he's still an antagonist.