That's a tricky question to answer, as much "modern metal" is technically quite challenging both left and right handed, not to mention the fact that quite a bit of it is detuned, or even 7-string. Slipknot and KSE both write really technical (and fast!) songs.
Tool is one of my favourite bands, and they have some great songs to play, though not too complicated. I'd go back to the Undertow album and look at songs like Undertow, Crawl Away and 4 Degrees. Not very technical, but great fun and great riffs.
If you're after something a bit more challenging, how about Redneck by Lamb of God? It's real work out for the right hand with some fast rhythmic picking and palm muting. All of these songs are on a 6 string in drop-D, so easy to manage.
For something a bit more proggy, I love the Soen album Lykia. The first song Sectarian is great - really melodic but chunky at the same time. If you need a hand figuring out the notes, Songsterr.com is really useful.
Have fun!
Matt
