Also, I think you mean their peak was from 1988 - 1991, OP. But, if we're measuring by US charts for the songs:
That gives you: (4) #1's: The Look, Listen to Your Heart, It Must Have Been Love, Joyride (2) #2's: Dangerous, Fading Like a Flower (1) Top 20: Dressed for Success (2) Top 40: Spending My Time, Church of Your Heart (1992)
(Sleeping In My Car, and How Do You Do! fall outside that parameter)
This pretty much covers their stateside success. Back then, all that mattered were Top 40's. That would have been from just two albums (9M and 11M copies WW) and "Listen to Your Heart" off of the Pretty Woman soundtrack.
If you shift the parameters just one year back, 1987 - 1990, Madonna would blow them out of the water. But, even sticking with 1988-1991: (3) #1's: Like a Prayer, Vogue, Justify My Love (2) #2's: Express Yourself, Cherish (3) Top 10's: Keep It Together, Hanky Panky, Rescue Me (1) Top 20: Oh Father
Like a Prayer sold 15M WW, I'm Breathless 7M WW, The Immaculate Collection 30M WW. She still seems to have fared better.
With Janet, Control was 1986 and janet. was 1993 (both huge successes), which just leaves the monumental piece of work that is Rhythm Nation:
(4) #1's: MIss You Much, Escapade, Black Cat, Love Will Never Do (2) #2's: Rhythm Nation, Come Back to Me (1) Top 5: Alright
12M WW. A staggering success.
Great poll. Nice memories. Roxette seems to have faded like a flower, though.