Reddit mentions of The 99 Most Essential Choral Masterpieces

While not in the least bit comprehensive, Amazon has a sale on the 99 Most Essential Choral Pieces for $2.49. It's a good starting place, I think.

My personal favorites have always been requiems - Mozart's in Dm is a pretty definitive one (the Dies Irae in that gets used A LOT in movies). Faure's Requiem in Dm as well is one that I think is just gorgeous. And of course everyone's familiar with Carmina Burana - even if you don't know the title of the work, O Fortuna is used a lot in pop culture. And it is a stunning, powerful work.

Also I'm a big fan of choral pieces in operas - Gounod's Le Damnation de Faust is SPECTACULAR, and the Metropolitan Opera did a production two years or so ago directed by Robert Lepage (one of my favorite directors of all time - he's currently doing the ring cycle, which I haven't been able to get tickets to despite knowing three of the designers personally and every time I watch the promo videos for it I cry because it is so incredible). Anyway, their videos are available on the Met HD player, and I would recommend whole-heartedly to check it out - he's a beautiful director, and Gounod is just amazing.