I assume you mean bands that not only come from germany, but who also sing in german, right? (Otherwise it would be quite pointless for learning the language ^^)
Well, the standard-answer to such a question
, Rammstein is of couse always a good start.
Probably one of Germanys most internationally well-known bands. If you like them, you may also like
Oomph! - not quite as well known, but were actually one of Rammstein's main sources of inspiration.
A good german-singing metal band imo is
Equilibrium - good luck understanding any texts though, I sure as hell can't most of the time! ;D
Eisregen are a little bit easier to understand, but not much.
If you like it a bit softer,
Subway To Sally might be for you. They are kinda soft metal/ folk inspired/ hard to classify...
Die apokalyptischen Reiter are also quite good
. Knorkator are always a lot of fun to watch and especially amazing live - they make awesome metal-parodies and other fun stuff and are just generally crazy. (Only funny if you already understand the texts
).
Other bands that come randomly to mind (not necessarily Rock/ Metal...too lazy to look up the exact genre now):
Megaherz (be a bit careful with using the words you learn through their music in class though
),
Das Ich (Whatever genre that is...they call themselves "Neue deutsche Todeskunst". Inspired by Gottfried Benn's poems.) and
In Extremo. (Medieval Rock. Or at least they were Medieval Rock last time I checked. Nowadays they seem to be more into...steampunk? Not sure, haven't followed them for a while now.)
Edit: Okay, one more. Not to be confused with Eisregen:
Eisbrecher