Good popular/mainstream manga (that are good) include Fullmetal Alchemist, for action/adventure and magic. Rurouni Kenshin for kickass samurai action with mild romantic sideplots. Bleach for action, supernatural, occasional hilarity.
Id suggest Berserk to anyone. Its dark and gory with romance, power, betrayal, magic, gods, and demons. Some of the religious themes make for some deep thinking. If you get offended easily when Christianity is insulted I wouldn't suggest it I suppose.
Its just amazing. Its also better than the anime ten fold.