It's not so much Link's dominance as Zelda's in general IMO. Last 10 contest winners:
1. Link vs. Ganondorf
2. The Legend of Zelda: Majora's Mask
3. Link
4. The Legend of Zelda: Ocarina of Time
5. Link
6. L-Block (Link was runner up)
7. Link
8. The Legend of Zelda (series)
9. Link
10. Sephiroth (Ganondorf was runner up)
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Who knew the Rivalry Rumble would be so predictable? SuperNiceDog, that's who.