I'm not that wise when it comes to romance mangas, but one I do recommend is Cardcaptor Sakura, which is probably one of the first series I ever really liked. :D
If you're looking for a manga that hasn't/won't be adapted to anime, I suggest Yotsuba&! since it's fairly new and simply hilarious, but then again, there's little to no romance involved.
Other than those two, I can't really recommend any more romantic/non-anime mangas, since I'm not exactly a huge fan of romance and I know little to no mangas that aren't made into anime...sorry. :(
BUT...with all of that aside, some very general manga I do recommend reading would probably be; Lucky Star, Hellsing, Azumanga-Daioh, xxxHolic, GTO, BLAME!, Monster, and Ikki Tousen, among others. Hope that helped. :)
EDIT: Oh, I forgot two very important romance mangas. o0 Chobits is a great romantic/comedy type of manga I'd recommend, except its also an anime, but still great nonetheless. And then there's She Monkey, which is also a romantic/comedy manga which I don't think has been adapted to anime yet. o0