sci-fi, out of this world, apocalyptic: we've got Alita/Gunnm for you: http://en.wikipedia.org/wiki/Battle_Angel_Alita
definately a well researched, actionpacked manga. One of my personal favorites.
The thing is, there are just too much out there and you should start with a few to get you going.
A nice mix of humar, action, art manga, which are easy to find. I'd recommend (male) beginners is something like this:
-Battle Angel Alita/Gunnm (scifi, apocalyptic, action)
-Rurouni Kenshin (medieval, actionpacked, humor)
-GTO (humor, modern)
-Love Hina (base romantic slapstick)
For more, you can browse at tokyopop. http://www.tokyopop.com/shop
But these four are famous and easy to find in the store and are really worth the money (and if you by any chance don't like them, easy to sell).
If you really really want to try them out before buying... there are scanlations too. Although these are hard to come by for these titles. A code of honour for scanlators is, that when a series is licensed and published in english, they will stop distributing. But there are always strays...