If by Hellsing you mean the normal episodes, you should definitly watch Hellsing Ultimate! It follows the manga and is much, much more awesome in every aspect.
Also, since you watched (and i just assume loved, since they are awesome :P ) Code Geass, Steins Gate and Death Note, Zetsuen no Tempest might be up your league. It has the same intellectual feeling and really keeps you on your toes. The first few episodes may seem too generic, but episode by episode it just keeps getting better and better.
Also, in regard to Naruto, Bleach, Fairy Tail and so on, Ao no Exorcist might also be something for you.
Now for myself, i am also looking for some new Anime. This is what i have watched so far, rating included to make it easier to recommend something:
http://myanimelist.net/animelist/Comburo