It's annoying to give out generic suggestions for anime as a whole without a certain desired genre or aspect specified, especially when you give so few details about yourself. This is partly because I can't give a very good reason for you to like an anime without sufficient knowledge. What aspects of
Shakugan no Shana led you to like it? How many anime have you seen? Do you have an MAL or AniDB account that you can share with us?
Nevertheless, I'll give you a few anime I greatly enjoyed that are generally liked in anime communities:
Puella Magi Madoka Magica
Fantasy thriller/drama with magical girls.
Cowboy Bebop
Sci-fi, episodic space western. Entry level stuff.
Fullmetal Alchemist: Brotherhood
Action fantasy. Please don't mistake this for the inferior 2003 anime
Fullmetal Alchemist.
Toradora
School romantic comedy.
Monster
Mysterious and dramatic psychological thriller.
What I also suggest you do is follow
this link, as it has a lot of charts on anime that are usually considered good. It can serve you well in the future.