You asked for classics... so you've probably read those already ^^' I haven't read many "old" shoujo manga but here are my faves:
https://www.mangago.zone/read-manga/fruits_basket/
https://www.mangago.zone/read-manga/ouran_high_school_host_club/
And for non-shoujo oldiegoodies, here are some:
https://www.mangago.zone/read-manga/yu_yu_hakusho/
https://www.mangago.zone/read-manga/rurouni_kenshin/
https://www.mangago.zone/read-manga/family_compo/
https://www.mangago.zone/read-manga/hellsing/
https://www.mangago.zone/read-manga/dragon_ball/
https://www.mangago.zone/read-manga/inuyasha/
https://www.mangago.zone/read-manga/jackals/
https://www.mangago.zone/read-manga/trigun/ and https://www.mangago.zone/read-manga/trigun_maximum/
https://www.mangago.zone/read-manga/wish/ (a good introduction to the CLAMPverse)
I personally love Skip Beat! It’s ongoing but it started in the early 2000s. The art style progresses a lot over time
gakuen alice! such a classic.
Feel free to check out my lists for this! I got a bunch of shoujo you may like (:
https://www.mangago.zone/home/mangalist/2085943/
Do you guys have any classic shoujo manga recos? Like Parakiss, beauty pop, strobe edge etc. Or even if it's not shoujo. Just oldies but goodies 90s-2000s manga TIA mwa