Aishiteruze Baby - I'd say its worth reading even if you've already seen the anime - 7 volumes
Miriam - the art is kinda old... but it's a
cowboy shoujo manga, which is just weird and probably worth reading just because its so different, lol - 7 volumes
Nana - I actually like the anime better for some reason - 17+ volumes\
Fruits Basket - awesome manga... more just shoujo than romance... well sort of - 20+ volumes
Here's a few where the romance is so minor a part of the manga they can't be called a 'romance' manga, but I like them much nonetheless (enough that I've reread them ^^):
Angel Densetsu - 13+ volumes
Kiseijuu - 10 volumes
Perfect Girl Evolution - 13+ volumes
I'm of course skipping many good manga in both categories because of the 5 volume minimum
. (this is turning into a jedinat's manga recommendation thread it seems ^^)