Second for East 1999. Doggystyle would be 2nd on my list. Bone’s earlier offering, “Creeping on a Comeup” is great too.
We all know that Aquemini is the best Outkast record by far, right? 
It’s still my favorite. But I’m a bit of a dilettante when it comes to hiphop. I came to it late; it wasn’t until I saw the “Lucas with the Lid Off” video in 1994 that I finally found my way into what was for me, until then, a pretty inaccessible style. Ill Communication, that same year, completely changed my world: I probably listened to it 500 times that year. Since then I’ve accumulated a lot more hiphop, but listening to most pre-1994 stuff still feels like homework to me. I consider it a gap in my education.
My personal favorites are the aforementioned, plus
Sole
Missy Elliot
Sage Francis
Atmosphere/Slug
CeeLo Green
Del tFH
Jurassic 5
Madvillain
The Streets
Talib Kweli
I’ve got to go with VCO3 on this one. Aquemini is easily Outkast’s best record, and this comes from someone who loves all the others, even the newest ones. It’s just so cohesive and streets ahead of it’s time, especially considering it was '98 when there wasn’t all that much other good stuff about.
I’ll second this. It really is that good. I’ve been listening to it nearly constantly for 6 months now and it’s still. that. good.
Others I’ll mention -
Wu-Tang - Enter the 36 Chambers
The Roots - Things Fall Apart
A Tribe Called Quest - The Low End Theory and Midnight Marauders
Gza - Liquid Swords
Raekwon - Only Built for Cuban Linx
Notorious B.I.G. - Ready to Die
For me, it’s De La - “Stakes is High”. I am obsessed with that album.