Rainbow had the potential to be a great album but the era itself was cheesy, probably because the industry in general was cheesy in 1999 due to bubblegum pop explosion.
She looked flawless. I'll never get the unbathed thing. Sure, she looked messy in the CTTA video, but she looked perfect in every other occasion. But still, she bathed in the Crybaby video, so there's that
Rainbow era was her biggest soap of freedom. It wasn't my favorite album, because some tracks are fillers with outdated production. I adore her live performances with "Can't Take That Away (Mariah's Theme)". She wanted so much for this composition, but Columbia's ego was really big for Mariah's creative vision. Also I really like her style, because she looked nature and fresh.
Also during the Rainbow era she was at her peak. All these special awards and wonderful interviews. Her speech for 1999 Billboard Music Awards, when she received Artist of the Decade. >
My personal favorites: "Heartbreaker", "Thank God I Found You", "Petals", "Rainbow (Interlude)".
Rainbow has many great songs and many songs that I can't stand. However with Rainbow she also released my favorite song of hers, Petals. Petals made me a stan and is one of my favorite songs of alltime. The lyrics are the definition of perfection especially in combination with the Rainbow Interlude.
Petals + Rainbow Interlude, After Tonight, Thank God I Found You
Her high notes alone. >
"Can't Take That Away" with all this promotion could be the biggest #1 from Rainbow. Can't believe it wasn't released for radio and main physical single.
On #1's: I still don't know if I should buy #1's or not. I love all the 4 new tracks in there, but I never buy GH since I don't like to own songs twice. Furthermore, she is releasing and uptaded version of the album this year, and maybe I will buy that...
On Rainbow: Petals>>> Your faves careers (even though I started caring for this song a few months ago)
masterpieces: Heartbreaker, Heartbreaker remix, Bliss, Crybaby, Thank God i found you, Petals, Take a look at me now (solo)
mediocre: Mariah's theme, After tonight (Really Miss Warren?? Your worst songs with Mariah???),
How much, Take a look at me now (with Westlife), Thank God i found you remix
Even Did I Do That? is cute 
And the best song of the album 
Sure, she looked messy in the CTTA video, but she looked perfect in every other occasion. But still, she bathed in the Crybaby video, so there's that 
, After tonight