I think the first four pretty much speak for themselves.
The j., UB, AFY is the only section where things get debatable. "janet." isn't too much better than AFY, and UB is better than AFY. By inference, UB is right on the heels of janet. AFY probably has more standout tracks than j. and UB, but they work better as albums, whereas AFY is more like a great collection of songs. "janet." is not that strong without its singles. AFY and UB are, i.e., TAT, Truth, BE, AYF, and Night. The biggest, and most obvious qualm about j. is the length. It starts to drag in the middle (TT - BOL). The only songs that save the album post If are WAYN and ATAP. But overall, the concept and direction of the album are clear, same with UB. The songs are a little better too. So that's why it's above UB, but not by much, with AFY not far behind UB.
Disc is definitely better than DJ and 20YO. Ya'll really give DJ too much. It's a watered down janet., lbr. She whispered her way through the album, didn't even write the songs, and the concept isn't developed much. DJ is full of good to mediocre tracks, no real great tracks imo. 20YO is just Janet trying too hard to adapt to that R&B/Hip-Hop blend trend that was going on during that time. DB, Enjoy, and TC are really the only tracks that "feel" like a "Janet song". Listening to it, it doesn't feel like there was a direction or idea for the album. What both albums have in common is that none of the songs have any real replay value, or the tracks don't really stick out for lack of a better phrase. Whereas with Disc, the album punches for the first couple of tracks just like previous albums before coasting into the slow jams, and then finishing with the mid-tempo tracks (TVR & AFY namely). FB - 2Nite is a much stronger opening set of tracks than DJ and 20YO. The only "eh" tracks are the awful GX and NLG. The concept of discipline is more pronounced throughout the album.