What is the proper pronunciation of "renaissance"?

It depends on whether you're speaking American or British English. Here in the U.S., we place the main stress on the first syllable and a slight, secondary stress on the third. The British have tended to stress the second syllable. It's more or less the difference between REN-nay-sahnce and Re-NAY-Sahnce:

http://www.dictionary.com/browse/renaissance

Here's an online audio pronunciation from American Heritage Dictionary, which is how I've always said it:

http://dictionary.cambridge.org/us/pronunciation/english/renaissance

My Webster's Eleventh gives six pronunciations, the last of which, naturally, is Frankie's British, this being an American dictionary. The next to last is the above. All the rest have the accent on the last syllable and the schwa sound in the second syllable.

Take your pick!

Actually, we use two different pronunciations in the UK. A renaissance of interest in mushroom-throwing would be pronounced re-NAY-sans, but a Renaissance Man would be REN-uh-sans. The second is closer to the original French.