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The minister beat memorable lines by Man Booker Prize winner Eleanor Catton and chart-topping teenage singer Ella Yelich-O'Connor, better known as Lorde.
Mr Williamson's winning quote received 45 per cent of the vote in a Massey University-run competition.
During the final reading of the same-sex marriage legislation in April, he said: "One of the messages that I had was that this bill was the cause of our drought. Well, in the Pakuranga electorate this morning, it was pouring with rain. We had the most enormous big gay rainbow across my electorate."
Mr Williamson said although he usually prepared his speeches, his contribution to that debate was unrehearsed and pulled together in the debating chamber. "I'd done no preparation at all, about four minutes and four bullet points on a scrap of paper - 'gay onslaught', 'celibacy', 'physics' and 'gay rainbow'."
The response had been overwhelming. "I seriously didn't expect it would make the Howick & Pakuranga Times. I've been told that it's played in 59 countries, been translated into 16 languages. I've got the Dutch, the Spanish, the German, the Swedish version on my iPad."
Massey University speech-writing specialist Heather Kavan said the speech was irreverent, visual, disarming and witty, but it was also emotionally moving for those affected by the marriage equality bill.
She noted that it had global impact, including 1.5 million views on YouTube and endorsement from high-profile individuals such as Stephen Fry and Ellen DeGeneres.
Political quotes took the top three spots in the competition.
In second place was an anonymous quote which was repeated on social media during reform of the Government spy agency: "The GCSB, the only government department that will actually listen to you."
It was the second year in a row a National MP won. Last year, Social Development Minister Paula Bennett's "Zip it, sweetie", a retort to Labour MP Jacinda Ardern, got the most votes.
Top 10 quotes of 2013 in order of number of votes:
1: "One of the messages that I had was that this bill was the cause of our drought. Well, in the Pakuranga electorate this morning, it was pouring with rain. We had the most enormous big gay rainbow across my electorate." - National MP for Pakuranga Maurice Williamson in his speech to Parliament supporting the gay marriage law.
2: "The GCSB, the only government department that will actually listen to you." - Unknown origin but repeated on social media.
3: "What didn't he know and when didn't he know it?" - Winston Peters querying John Key's knowledge of the Parliamentary Service's actions.
4: "I'm not a spreadsheet with hair." - Auckland singer/songwriter Lorde.
5: "Why are you going red, Prime Minister?" - Kim Dotcom at the parliamentary inquiry into the GCSB spying on New Zealand residents.
"I'm not, why are you sweating?" - Key's reply to Kim Dotcom.
6: "Male writers tend to get asked what they think and women what they feel." - Man Booker Prize-winning novelist Eleanor Catton.
7: "If there was a dickhead that night, it was me." - National MP Aaron Gilmore reflecting on how he got intoxicated and called a waiter a "dickhead" at the Heritage Hotel in Hanmer Springs.
8: "In New Zealand nobody takes you seriously unless you can make them yawn." - Author James McNeish at the Auckland Writers and Readers Festival.
9: "That little ball of fluff you own is a natural born killer." - Gareth Morgan's Cats to Go campaign website.
10: "He's an extraordinarily lucky cat." - Massey University veterinary surgeon Dr Jonathan Bray after removing a crossbow bolt from the head of Wainuiomata cat Moomoo.
- Isaac Davison of the New Zealand Herald