Since I only play guitar on Rock Band, I can only vouch for guitar (sorry drummers, vocalists, bassists and keyboardist) I'll categorize my favorite by difficulty for guitar. Some of these songs are also from Rock Band and Rock Band 2.

Apprentice:

Rebel Girl by Bikini Kill
Difficulty: 1/5
Supervision Recommended
I believe my punk side is showing on this pick, it's a pretty straight forward song with a lot of chords and a cool little solo. I love to warm up with this song, and I'm not going to lie: I sing along to the song as well.

Day Late, Dollar Short by The Acro-Brats
Difficulty: 1/5
Supervision Recommended
Again my punk side shows, what can I say? It's a great song to sing along to and play with. Some alternate picking is required during the verse, but I'm sure it's nothing you guys can't handle.

I Was Wrong by Social Distortion
Difficulty: 1/5
Supervision Recommended
Another punk song by one of my favorite bands, really fun to play.

MidLife Crisis by Faith No More
Difficulty: 1/5
Supervision Recommended
Ok, so I lied about not vouchering for other instruments, I don't have a mic but I sing along to this song all the time. MidLife Crisis is an amazing song by an amazing band. (This isn't DLC but I really enjoy it)

Solid:

We Didn't Start The Fire by Billy Joel
Difficulty: 2/5
Supervision Recommended
My favorite song by Billy Joel, I highly recommend this song for everyone to get.

A Looking In View by Alice In Chains
Difficulty: 2/5
Supervision Recommended
One of my favorite songs from Alice in Chains new album, this one is a bit long clocking in at seven minutes, but I can get enough of it.

Limelight (original version) by Rush
Difficulty: 2/5
Family Friendly
Can't really say anything about this song, it practically speaks for itself: A Masterpiece.

Moderate:

Hey Man Nice Shot by Filter
Difficulty: 3/5
Supervision Recommended
A powerful hit from Filter. With some intense strumming during the chorus, with one of the best bass lines I've ever heard.

Gay Bar by Electric Six
Difficulty: 3/5
Supervision Recommended
An amazing guitar riff with some hilarious lyrics, not to mention a great solo.

Would? by Alice in Chains
Difficulty: 3/5
Supervision Recommended
One of my all time favorites by Alice In Chains, with a heavy duty riff at the end that makes the entire song well worth it.

3's&7's by Queens of The Stone Age
Difficulty: 3/5
Supervision Recommended
My favorite song by Queens of the Stone Age, featuring some amazing solos and some serious fret work.

Bad to The Bone by George Thorogood&The Destroyers
Difficulty: 3/5
Family Friendly
I grew up listening to George Thorogood, such an amazing guitarist. I hope you got your fingers warmed up by now, because it's going to be tough.

The Kids Aren't Alright by The Offspring
Difficulty: 3/5
Supervision Recommended
A smash hit by The Offspring, with some catchy riffs and a great solo.

Challenging:

Fell On Black Days by Soundgarden
Difficulty: 4/5
Family Friendly
One of my all time favorites from Soundgarden, definitely pick this monster of a song up.

I Believe In A Thing Called Love by The Darkness
Difficulty: 4/5
Supervision Recommended
A monster hit from an amazing band. Featuring some catchy riffs and a boat load of solos. This song is always in my set lists.

Jailbreak (live) by Thin Lizzy
Difficulty: 4/5
Supervision Recommended
My favorite song by Thin Lizzy. Amazing guitar work that's sure to set your fingers ablaze.

Nightmare:

Shipwrecked by Alestorm
Difficulty: 5/5
Supervision Recommended
A nice hidden gem from the RBN, definitely pick this one up. Pirates for the win!

Bulletproof by La Roux
Difficulty: 5/5
Family Friendly
Definitely my guilty pleasure, I can't get enough of La Roux. Featuring some serious fret work and amazing lyrics.

Paradise By The Dashboard Light by Meat Loaf
Difficulty: 5/5
Supervision Recommended
A classic from the legendary Meat Loaf. This song is really long, clocking in over eight and a half minutes. Get ready for a work out on this one.

2112 by Rush
Difficulty: 5/5
Family Friendly
The legendary song from Rush, a twenty minute long masterpiece that is sure to set your tendons on fire.

Impossible:

The One Who Walks Alone by Fallen Angel
Difficulty: Skulls
Unrated
Another hidden gem in the RBN, definitely my all time favorite song to play on Rock Band. Amazing lyrics, epic guitars and a solo to melt your face off.

Hallowed Be Thy Name (Live) by Iron Maiden
Difficulty: Skulls
Supervision Recommended
An epic song by an epic band, with blistering solos and overall amazing musicianship.

Through The Fire And Flames by Dragonforce
Difficulty: Skulls
Supervision Recommended
A tendon shredding, finger breaking, face melting song by Dragonforce. I highly recommend you put on no fail mode. Because this song will make your head explode.

Well, these are just a few of my favorite songs. Go check them out when you get a chance. What are your favorite songs on Rock Band?