How this thread exists and doesn't have the word "MARS" in it is beyond me.
Mars is my favorite manga of all time, and is completely released by Tokyopop. It has amazing characters, great story, and I just love it so muchs
15 volumes + 1 prequel.
Um, also Maison Ikkoku is a must. Great comedy and great drama with amazing characters. I'd argue the precursor to all the romance manga being released here today. 15 volumes, all out by Viz.
In terms of stuff coming out now, Sand Chronicles, Honey and Clover, and Nana all are must reads.