I decided to update my list. Click the spoiler to see what I posted earlier:
Spoiler Alert! Click to show or hide
Most of my favorite romantic manga are ones that I discovered through their anime adaptations:
Spice & Wolf (though the novels are better)

Toradora! (though the novels are better)

Otome Youkai Zakuro
and my favorite of all time:
Oh! My Goddess!

Since then, I have picked up two new romance manga that I really love, and one that I like.
First, the one that is only at the "like" level: Photo Kano: Sweet Snap - based on a visual novel, the girls all look a bit too much alike, and they all have the same, very well endowed figure, though not outlandishly large breasted, it is still not my favorite style and there are far too many pointless panty shots. But it is about a kid that is into photography, and the main couple seems interesting.

Of the other two, one of them got an anime this season: Mysterious Girlfriend X - I picked this up when I heard about the anime adaptation and fell in love with it! It is quirky, a little kinky, and ultimately, surprisingly sweet!
Sayonara Piano Sonata - This is a real find, in my opinion. It is based on a series of light novels (which finished at 5 novels btw. The pictures below are actually the light novel covers...) The manga is fun too, but the light novels are really quite special. It is a fun story with lots of musical references, so many that it probably will never get an anime (there is even a comment to that effect in the omake for the first manga volume! Copyright issues would be a nightmare!) The story is fairly typical harem comedy with an obvious "right angle pair". The main female character, Mafuyu, is one of the most amazingly drawn characters around! Half-hungarian, half-japanese, with maroon colored hair (it is called that in the translation... I believe she is really supposed to have hair that is a dark shade of light red, which, in certain light, really looks maroon. The images on the covers have a fairly realistic color of red, at least.) She is a classical piano prodigy who suddenly disappeared from the perfomance stage two years before the start of the story. Our protagonist Nao's task is to find out this angelic looking by very ascerbic tsundere's secret reason she quit performing, help his childhood friend and sempai to form a band, and maybe, somehow, figure out that there are three young women totally head over heals for him, along with finding some purpose in his own life. Love, Rock & Roll, and classical piano: what a combination!