I have been playing guitar now for about a year and was wondering the best scales and modes for metal songs and jams
minor pentatonic, harmonic minor, aeolian (aka minor) and phrygian. those are some good ones, but in the right context you can use just about any scale in metal
Diminished in any form is fun, but there's no specific scale that makes something any more metal than something else. Its more how you use it, and what tonality you want.
Harmonic Minor
Melodic Minor
and some locorian and phygarian (sp?)
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Lol. Locrian and Phrygian also, Phrygian dominant is pretty cool. Its the 5th mode of the harmonic minor scale. Try the whole tone scale too, it sounds pretty exotic if its phrased well.

The best thing I can tell is to learn to phrase things well. If you know every scale in every mode in every position, but you can't phrase things well, you're in trouble. It'd be like having a 500,000 word vocabulary and not knowing any grammar at all.
Try the altered scale, hehehe.
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if you dont know your major scale yet id really suggest learning that before anything else :P

argghh i hate altered chords ><
Scales don't give the sound. The phrasing does.
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Ok now seriously.

It depends on your playing style and what style you play. For example a lot of Thrash songs have a Blue Scale solo on em. Death Metal focus more on Chromatic/Aeolian/Harmonic Minor Scales(Thrash too) Phrygian is also popular.

It depends on your mood and song structure. See I am a Thrash Guitarist my self so honestly Ill just go with something that fits my style and sounds good along with the rythm parts and riffs.

You can also experiment. Chuck Schuldiner was known for using his own scales and stuff than then came to be actual scales. I recommend that you just listen to a lot of music and especially the metal subgenre in which you specialize. So you get to know what will go better with what you write and stuff.

I actually like to use a lot the Aeolian Scale for some reason. Now Im going to work on learning more scales to improve.

Thats what I suggest you should do.
use the appropriate scales depending on whatever progression you are playing over?