Neo-soul was too limited of a niche (in terms of audience) to produce any universal classics.

B'Day is a possible retrospective classic, kinda like Reasonable Doubt in rap. The smartest move she ever made was to re-release the album in conjunction with the multiple videos. That definitely elevated singles like "Get Me Bodied" to classic status, and brought more fans into the fold.

I don't know if it's considered true R&B, but Timberlake's FS/LS is up there too. The core urban R&B audience may not have embraced it as a whole, but it's impact on the larger mainstream audience can't be denied.

I feel like there should be at least one mention from the BadBoy R&B stable, as well as an early Timbaland-helmed project. But I can't think of any off top.