#1
I'm at a bit of a song plateau right now. I can't seem to move forward in my techniques and playstyle, so I'd like some songs that would help me either improve my techniques, or just become more fluid in my playstyle. I'm currently trying to learn the Paranoid solo and Animals by Muse, but I can play stuff like Green Day, GnR, Marylin Manson, Iron Maiden etc.

Oh yeah, preferably less heavy metal songs, as I'd also like to play these songs to my friends and they aren't quite metalheads like I am.
#2
You gotta be more specific. Challenging is very vague and is based on skill set. There are many styles of guitar playing across many genres of music and they all require different levels of proficiency with different techniques. To one person, Jerry's Breakdown might be an easy piece to pick up while a Ramones song would be tricky (likely someone with no experience flat picking). To an acoustic flat picker, fiddle tunes like Black Mountain Rag and Beaumont Rag might be no problem but something requiring even simple finger picking might be very difficult. To a novice player, just about anything could be challenging.

Given that you didn't really specify what exactly you were looking for, I'm going to just recommend learning to play Hot Wired by Brent Mason.
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#3
If you're serious about guitar, you need to learn some Stevie Ray Vaughan at some point - his cover of Little Wing will escalate your playing enormously.

Also learn some Hendrix - it changes the game when you get some of those tunes under your fingers.